Interesting thread - it's always nice (and sometimes quite helpful) to see how other people manage a bunch of computing devices.
Mine:
1) 17" SB MBP (8GB RAM, OWC SSD + 500GB HD; no optical); my personal primary machine, used almost exclusively on the couch for general media consumption (web surfing, mostly), writing (as in text), light-duty work (web/image processing), and occasionally taking elsewhere.
2) 27" 1st-gen i7 iMac (12GB RAM, stock HD); my wife's main machine for everything she does, and my personal machine when doing color critical work (because of the IPS screen), or anything that needs the extra RAM/bigger screen, although it's otherwise not any faster than my MBP.
3) 2009 franken-mini (4GB RAM, both internal SATA ports rerouted to eSATA, multiple external drives attached). This acts as an HTPC (it's hooked to the TV and sound system), iTunes server, a home file server for most household data, a server for TM network backups, and a place to run bittorrent or other large downloads when I do them, so I don't need to leave other computers on.
4) AEBS gigabit, hardwired to the iMac, Mini, modem, and with an extra cable if I want more speed on the MBP or am working on somebody's computer. Everything else (including printer) is wireless.
5) 3rd gen iPod Touch 32GB; lives in my coat pocket, used for portable music, occasional gaming, and PDA-type stuff.
6) B&W old-school iPod 20GB; lives in the car glove compartment hooked to the stereo for driving music. (I like this, because it's so old that if the heat fries it or somebody steals it, I really don't care.)
7) iPod shuffle that I got free with miles and have never really ended up using.
The management of all of the above is the tricky part, and I'm shuffling stuff around and working out some kinks after upgrading to Lion. Mainly, though, my strategy is to keep EVERYTHING on the Mini, and either access it directly via the local network (since I rarely compute away from home) or synchronize it via periodic connection.
Almost all of my data (photos, writing, web projects, etc) lives on the mini server; this is handy because it doesn't really matter which computer I'm sitting in front of in terms of using files, and since it also has a connected backup drive, it backs up everything important automatically and without additional effort. The only downside is that access to big media files from my laptop is a little slow due to wireless, and of course if I'm not at home I only have what's on the laptop, but I rarely use it away from home. (Actually, I don't even really like laptops all that much, it's just I prefer sitting on the couch instead of at a desk.)
Another advantage of having almost everything on a server is that I don't have issues with too much data for a laptop (even with SSD) or needing external drives on the iMac. If I run out of space, I just add or replace a drive attached to the mini and am good to go. (My eventual goal is to get a TB-equipped mini and a 4-bay TB enclosure to run as a RAID5 and store all my data plus DVD rips on it, instead of a stack of 2TB bare drives with a dock for the DVDs, which is pretty awkward, but I can't afford that yet.)
The Mini has a wireless keyboard and mouse for when I'm using it to watch videos on TV, but otherwise is controlled almost exclusively via VNC from one of the other computers, usually headless (without the TV on). (Lion has opened up some cool new possibilities with VNC, but it's currently kind of flaky; I'm hoping 10.7.1 fixes some of the kinks I've been having with VNC control.)
Music is exclusively hosted from the Mini, which works great since it can play through the stereo without needing any computers on, and you can use the iPod Touch-based remote for full access without needing to even turn the TV on. I plug in the two iPods occasionally (the one in the car very rarely) to sync (which, again, doesn't even need the TV to be on, since it happens hands-off). On the rare occasion I want to listen to music from one of the computers, accessing the network library takes care of it.
The only downside to having a mini as an iTunes hub is that when you're shopping for music or apps, you need to either do so on it directly, or buy stuff and then connect to the mini and download the stuff there, too. (I usually do the former, though sometimes I shop on a local copy of iTunes then just connect remotely to make the purchase and download.)
All my mail is now set up as IMAP, so that keeps itself synched between both computers when I need it.
I have address book sort-of-hosted on the Mini, and synching through online contacts, which keeps my computers and iPod up to date automatically, but is still a little bit awkward; I'm looking forward to iCloud to take care of that.
The only thing left is bookmarks and user-space settings, which are my weak point. I don't much mind my prefs being different between desktop and laptop (since I use them for different purposes), but I haven't come up with a good way to sync bookmarks between the two that fits the way I work and organize bookmarks. Part of the problem is that I use three different browsers (each for a different purpose), and nothing seems to work smoothly with that. Again, I'm hoping iCloud will help with this situation.
