Personally, I think a 27" iMac and an 11" MBA is the perfect combination. I keep all of my "documents" in dropbox, so they are always accessible and syncing wherever I am.
I do not keep any music or video on my MBA. All of that lives on the iMac. I keep a copy of 100% of my music on my iPod which is with me when I travel. I keep a small selection of about 20 or so movies on my iPad for use during trips. I seldom watch more than 1 or 2.
My iMac sits at home, and contains the original copies of 100% of my data. As I create new documents, or make any changes to existing document on the road, they instantly sync back to my home iMac via dropbox. From there, it is double backed up locally (Time Machine) every hour... and to the cloud (Crashplan+) every 15 minutes. Even if I lost all of my computers simultaneously... I could easily recover 100% of my data.
For photos... I run Aperture 3 on both my iMac and my MBA. A3 is specifically designed to allow you to check projects into and out of a library... so it is dead simple to create new projects on the MBA when in the field (or on a family vacation)... and then migrate the project onto the iMAC when I get home. CP+ also keeps my MBA A3 library backed up to the cloud when I am traveling. Hence... it is virtually impossible for me to lose any pictures when in the field.
Personally... I think that an iMac and a MBA is the perfect combination, especially if you use the cloud to keep everything in sync. I completely stopped using my 15" MBP. I haven't turned it on in over a year. My wife inherited my Rev C MBA when I got my 11.6" model last October, and she has not turned on her 15" MBP either. Neither of us want to carry one anymore as we are spoiled by the lighness of the MBA. When the next MBA is released (presumably in the next couple of months)... I'll buy that for myself and my wife will inherit my existing 11.6" MBA. (It's great to have a spouse who is happy with N-1 technology... it allows me to justify upgrading every cycle).
The only significant change that I plan to make is to buy a 2nd iMac. My wife and I currently share the single iMac, which is leading to marital dissatisfaction because we both want to use it (that is a joke). Still, it makes sense for us to each have our own. We both use them too much to share.
Beyond that... the iPad is the perfect device for use during a flight, or for just having handy for quick access during travel. Back in the room at night, we almost always use our MBAs.
/Jim