...just to make sure I understand what you have set up: When you import video from your camcorder, you save the AVCHD file in a directory on your 4TB external hard drive. Then you use iMovie to "import" that video file (stored externally) into an apple-TV resolution movie in iTunes, which is the only version that is stored locally on your mac.
Yes, here's even richer detail. I use Chonosync to sync the camcorder AVC content to the 4TB external. Then, I manually, input just newly shot video into iMovie (from the 4TB), edit it to taste, then render it for

TV. The input for iMovie is in a folder called "Scratch" on the iMac's internal hard drive. The Scratch folder is also excluded from Time Machine backups. Once rendered, I often give it a quick check, then send it back to the "home movies" folder (organized by year) on the 4TB.
When Scratch starts taking up too much space, I just delete everything in it, knowing that some day, I'll reimport and re-render all that AVCHD to master copies of 1080p, at which point- after verifying I've got exactly the net footage I want at it's native 1080p resolution- I'll probably dump the AVCHD backup files, and those 1080p renders will become my master copies.
The 4TB drive is backed up 2 times, once to another 4TB, and then component folders of raw and processed content are backed up to 2TB drives stored offsite. I could just pick up another 4TB drive for my 2 backup approach, but I happen to have spare 2TB backups just laying around doing nothing.
Time Capsule is attached to the AEBS solely backing up the iMac SSD + 2TB internal drive, my laptop drive, and the other iMac in our house, but not "scratch" (folder) and not the 4TB attached to mine. This all works really well and doesn't involve much cost considering what I'm doing and the (family) perceived value of the content.
I chose this approach over using Time Machine for everything because that would take a small server (for Time Machine) and I generally think of Time Machine as good for all the smaller files like Applications, photos, etc, not as good for big multi-GB files like videos, where I might occasionally tweak the description or name or something in iTunes, and then Time Machine would back that up as a new copy.
My 500GB MBP hard drive is already 80% full, so I'm going to start by offloading the rest of my iMovie library to a different external hard drive, but I don't yet have a plan for keeping those backed up. (I prefer automated backup systems like Time Machine, because my forgetfulness doesn't impact the process.)
Everybody has their own way of doing things, but I do prefer to backup big-file content manually, mostly because the small tweaks can burn backup space quickly. However, if you are worried about forgetfulness, you might consider one of those 8TB raid boxes from OWC, to allocate as your Time Machine box. That would be plenty for backing up 720p and 1080p big files for a good long time given what you're doing. And again, not really that expensive considering how valuable that content will become as you age.
I just find that the phone video gets archived (via itunes backup) and shared (via posting to flickr, youtube, facebook, e-mail) much more quickly than video taken with the other two devices. I'm lucky if I get time to sit down and deal with the SLR/camcorder video once every month, as it takes me a couple hours+ in front of the computer to import, edit, sort, distribute to family/friends, and I find I have less and less time in front of the computer at home, due to baby's needs. I can edit/share my phone video while sitting on the couch for a few restful moments or even while lying awake in bed, on the phone itself.
I understand... and remember how it was. What I can offer is that it won't always be that way, so archive it now and get to it when you can. Maybe you won't have time to process AVCHD for a couple of years, but when the baby is off to school, and especially later on as they become more independent, you'll find more and more time for these kinds of projects. You'll be happy that you have a "ton of old footage" to go back and process than not ("because I never thought I would have the time to actually do anything with it"). Trust me on this one. I know first hand.
I still don't get why the movies need to be imported into itunes, though. I mean, apple tv can stream photos directly from iPhoto library without the photos being imported into itunes, right?
Not exactly, iTunes is involved in all things related to media pumped to

TV. It's not 100% clear to me exactly what goes on there, but I'm pretty sure that iTunes will convert copies of your iPhotos (you earmark for

TV) optimized for HD resolutions. Either they are stored in association with iTunes directly, or they may be stored in association with iPhoto. I'm certain that the photos stored in iPhoto are not the source of what appears as duplicate photos stored for/in

TV, so I know there's a smaller copy of the photos stored somewhere for

TV. Apple TV is an "ipod for your television"; just like your iPhoto library is not loaded into your iPod at full resolution (say a bunch of 10 MP images to show on a tiny iPod nano screen), these optimizations are also done for other iDevices. I'm pretty sure this is all handled by iTunes.
Also, that's another issue: I've actually gone through the rigamarole of exporting 1080p video via iMovie / Quicktime (it takes some extra steps to do it. . . found the instructions on some blog post), and the resulting 1080p movie file looked ok on my HD TV, but not as good as when I plugged my camcorder directly into the TV and played it. This made me lose confidence in the Apple software's ability to export a good 1080p video file, and helped to justify my not using 1080p. Do you have another method of watching your 1080p home movies on your HDTV other than plugging your camcorder into the TV? Apple certainly doesn't make it easy to get a 1080p video out of iMovie.
No, it does seem to be a problem to get an excellent copy out of iMovie at 1080p quality. There's tons of workflows and discussions online, but I've generally concluded that we haven't been given a version of iMovie yet (maybe 2006's iMovie HD) that is really set up for this. I always expect we're one version away from that capability (with processing for 5.1 audio too).
However, if not, when re-rerendering for 1080p in the future, I'll probably just go with Final Cut Express. But I do think iMovie will get there. Another reason to store the masters as AVCHD.
Also, the crowd that cares a lot about 1080p is probably going to be either a.) home video gurus like you with lots of home video 1080p content and/or blu-rays ripped legally to hard drives or b.) video pirates who get 1080p content via bit torrent. Group "a" will have little use for Roku, since they don't adequately do streaming from your home computer to your TV, and so Apple is likely to lose that group to Roku any time soon. (example: are YOU going to buy a Roku XDR? I'm guessing not.).
I intend to buy some solution for 1080p playback sooner than later (I waited for this new version from Apple in hopes that they would deliver on this long-requested feature). It may or may not be a Roku. Roku can stream from home computers, but it is also well suited to just attach storage to it. So I could just copy all my media to a spare 2TB, attach it to the Roku or similar, and that could be a solution. I'm not in love with Roku- only pointed to them to disprove the popular belief that Apple couldn't put 1080chips in a set-top box and still retail it for $99. Obviously, they could have done so.
In my own case however, I'm about to buy a newer HDTV. I'm looking at the latest Samsungs with built-in apps (like netflix):
http://www.samsung.com/us/appstore and normal USB ports for home media libraries. It may be my new

TV like option (minus a set-top box). As a matter of fact, tomorrow I'm taking a USB stick loaded with some test video rendered in various formats to Best Buy to check out how the playback of everything from old SD home movies to 1080p renders from iMovie HD (2006) and iMovie 09 all look. I hope it "wows" me.
Also, group "a" has enough income to support the many external hard drives to support the maintenance and storage of the big 1080p files. At $99, it's pretty clear the Apple TV is trying to be a mass-market-appeal device that is affordable to people with lower incomes as well, so group a is not the target audience.
Now here's the reality. My whole DVD collection plus all of my home movies rendered for the current

TV currently comes in under 1TB. When I re-render all the home movies to 1920x1080p, I expect that to increase to no more than 2TB. A 2TB external to hook to a Roku or Samsung USB jack can cost $120:
http://www.walmart.com/ip/Western-D...ner=jXot6eVeYJg&sourceid=22000130430528900513. So for about the same price I paid for the original

TV 4 years ago, I could cover this base in a complete way. The point is that it's not hugely expensive to go 1080p for these kinds of applications, even for a poorer player. If they can afford a 1080p HDTV, they can probably afford a few hundred dollars for some kind of 1080p source.
And here's a hint: export your movies all at "best" settings, then run them through Handbrake for optimizing them for h.264. Just last night a 1920x1080 render out of iMovie came in at just under 10GB. The same video pumped through Handbrake brought it down to a bit more than 1.2GB. Both versions look just about exactly the same on the (small 27") iMac screen. I haven't been able to compare them on a big screen yet. But I do know that handbrake really optimizes well, while iMovie doesn't seem to be set up for output optimization at the same level.
Finally, another marketing explanation: Apple has stuck its iphone's with 720p max res video. It is selling and renting 720p max/res video over iTunes. And Netflix is only streaming at 720p. Perhaps apple didn't want to announce their new device supports 1080p, and have the backlash be all the people saying: "I bought this device because it supports 1080p, and then I find that there is no 1080p content available for it! Lame!" The 1080p-content-using segment of the population is way to small to be driving Apple's short-term strategy.
Maybe so, but they could have included 1080p hardware and not announced it if they were concerned about that. Officially it would support 720p, but unofficially it would play 1080p. I'm awaiting definitive tests in hopes that that easter egg might actually play out (doubtful).
The path to 1080p content in iTunes for

TV only can be realized by getting 1080p

TV hardware installed in homes on a massive scale. When enough are in place, some Studio will be tempted to test whether 1080p content via iTunes will be profitable to offer as rentals or purchases. Any Studio that decided to run that test this week will see a massive failure, as no 1080p

TVs in place can not possibly prove whether we'll pay for 1080p content in a profitable way.
Thus, it's not really a chicken & egg situation. Apple can lead with 1080p hardware and let the software catch up, just like offering 4 & 6 core chips in Macs before all software can utilize them, or tethering capabilities in iPhones before AT&T software was ready, etc. However, it certainly won't work the other way (software before the hardware).
Besides, if this

TV is going to have apps, by the time the stealth 1080p hardware issue actually grew into something, sources with 1080p content like VUDU:
http://www.vudu.com/ could roll out their app to feed this little box. There's already 1080p content avaiable on youtube. Some vodcasts would go 1080p content if there was any way for it to be watched via

TV. And so on. In other words, if the hardware gets heavily entrenched thanks to your big Christmas sales theory, every additional unit sold adds incentive for many players to test the profitability of 1080p content. It wouldn't take long for some Studio to approach Apple wanting to test it as an additional option in the iTunes store- especially as a big base of owners are clamoring to rent or buy such content in a way that gets picked up in the press. Sellers always love to see hungry buyers ready to buy.
As is, Apple might sell millions of these 720p MAX units this Christmas. But if they do, not a one can prove if there's a market for 1080p content via iTunes rentals/sales. Not a one.