I still work with my 12" PB because it still has the reasons I bought it for: It's small and powerfull. I didn't even buy it because it was a Mac: I needed a small, portable computer for on the road. OS X would be fun to try, at home where the serious work is done I would still have a fully equipped desktop.
Needles to say, after a month or so I hardly ever used my desktop and was working on the PowerBook (with an external screen, granted) all the time. And now I'm finally looking for a replacement. Not for the PowerBook, which will remain my trusted travel-companion since there is nothing equally small and light available, and it still feels powerfull enough. But my dust-collecting desktop will be replaced, and it will be replaced with a brand new iMac. Yes, I'm addicted now
When do you plan to get your iMac btw?