30" iMac would be at my desk FULL TIME, a MacBook Pro 13" would be right beside it & a MacBook Air would be in my bag. the 13" would replace the tasks of my 17" as my portable power house, iMac would be my work computer and MBA would be skinny & sexy companion.
That's my opinion but having two laptops now is well worth it and they compliment each other perfectly!
That's close to my set up ... Mac Pro with dual 24" ACD's, a 13" MBP and a MBA...
The Mac Pro I use at my desk for heavier work, design /cad, downloading, back up, server etc.
I love my 13" MBP, small, light, powerful handles most everything I throw at it, just a GREAT computer, never hear the fan, doesn't even get hot under normal conditions ...
And, finally my little MBA, I love it too, while it's far from a main computer or adequate desktop replacement for my needs, it is the perfect netbook for me, so thin and light I take it everywhere. Most of my computing is done 'in the cloud' now so syncing between the three is easy, for files I use sugarsync couldn't be better, imo.