I personally think the iMac/MBA is the perfect combination. My general advice is dependent upon how many computers the person wants:
1 computer: 15" MBP
2 computers: iMac/MBA combo
For starters, having an iMac enables having a compeer with "unlimited storage" all the time. This is extremely valuable because you can implement the methodology that 100% of your data "lives" on the iMac. Any data on any other computer is a "temporary copy".
This means that you have a single place where it is "mission critical" to have a very robust backup system. I use dual overall backup on the iMac: local via Time Machine; cloud via Crashplan+. I also keep manual copies of my irreplaceable media (personal photos and camcorder movies) on a pair of external HDDs that I rotate between home and off-site (my work office).
Once you do that... you can create a mobile environment on your MBA where everything is automatically synced back to the iMac in real time, irrespective of where you are in the world. The few things that I find very valuable are:
- IMAP email
- iCloud Services (Contacts, Calendars/Reminders, Notes, Safari bookmark sync and iCloud data
- Music Match
- Witopia VPN
Dropbox is pretty obvious. Any content that I need when mobile "lives" in dropbox. If I create new documents etc on the road... they are stored in Dropbox. Hence... they are always synced back to the iMac (which is on 24/7) so they are automatically backed up back at home. This makes backing up the MBA less critical while traveling.
IMAP email is supported by most (but not all) email providers now. It gives a "common view" of your email from as many clients as you wish. Hence, you can delete, organize, or manipulate email from either the MBA, iMac, or devices... and the organization propagates everywhere.
1Password is generally considered to be the top password management system out there. I use the dropbox sync feature to keep the encrypted keychain in sync across every system and device that I use.
iCloud service continue to improve and I find increasing usage of the features. This is especially true as I've gone "all Apple". For example, I've shut off Xmarks bookmark synchronization and now just use iCloud sync of bookmarks among my computers/devices.
I am a Music Match subscriber... which gives me access to my entire music collection on both my MBA and devices. My "original music", like everything else, lives on my iMac.
Photography: I am an Aperture 3 (A3) user... and one of it's greatest features is the ability to migrate projects from machine to machine. I am not generally using cloud services to do this... but what I do is import my photography into A3 every night, so I can begin the steps of photo organization during the trip... or on the flight home. This is especially true when I take long trips (sometimes 2-3 months) and accumulate 1000's of photos. In addition to importing photos into A3, I never delete SD cards... and I always keep the SD cards separate from my computer... in case of theft. Then when I get home, I'll transfer A3 projects into my iMac, and after everything is backed up, I can erase the SD cards. I do not use "Photo Stream". It can work well for typical consumer use... but I consider myself more of an enthusiast and for the amount of photos I take, photo stream seems to get in my way.
VPN: When traveling, it is inevitable that you will be connecting to hotspots in hotels, airports, coffee shops, etc... which is akin to having unprotected sex with hookers. Subscribe to a VPN service or set up your own. I use Witopia which is fairly inexpensive and generally has good service. I NEVER EVER use any hotspot (outside of my home) without turning on Witopia or using my corporate VPN.
A couple of things I have not touched on:
Backup while traveling: I personally use Time Machine/Time Capsules at home for both my iMac and MBA... but I do not use TM while traveling. I think it is too cumbersome to carry an external HDD... and if you follow the advice above, then all of your data is automatically backed up to your iMac at home (I never shut mine down). There are exceptions. When I was on sabbatical and traveling for 10 weeks, I turned on Crashplan+ on my laptop... so that it was continually backed up during that long trip. I did this primarily as an extra step to protect my photos. However, for routine travel of a few weeks or less... I don't bother with backup... since all the data is continually synced back to the iMac where it is double backed up automatically.
Remote access to your iMac. There are a number of services such as LogMeIn, Teamviewer, Back to My Mac, that give you remote access back to your iMac. I am currently not using any of them on my MBA. I used to have LogMeIn, but I didn't like that it was always running in the background and if I turned it off... it would come back on at the next reboot.
Finally... for security purposes... turn on FileVault2 on both your iMac, and especially your MBA. This provides full disk encryption of your data. It is also wise to set a firmware password on your MBA which closes a minor vulnerability even with FV2 enabled. Also... it is best to power down your MBA when unattended, such as when you leave it in a hotel room. I also power down when I put it away in the hotel safe because I do not want the WiFi or BT radios blasting away at full power trying to get outside of the steel safe.
Bottom line: Try to think of creating a mobile environment that would pass the following test: If your MBA was stollen during a long trip... your data would be safe (i.e. you would have only lost a hunk of iron). Also... you could walk into the nearest Apple Store, buy a new MBA, set it up with just a few steps and a few cloud logins... and you would instantly have your entire computing environment recreated... including all of your mobile data. It is incredibly powerful (but something I hope you never need to do).
Hope this helps... no doubt I am forgetting some things that are very helpful. Enjoy your iMac/MBA combo... the ideal computing combination.