I have a PowerBook and was thinking of getting an iMac (and keeping the PB), but now I'm leaning towards getting a MacBook, selling the PB, and getting an external monitor, large external HD (7,200 rpm) and maxing out the RAM. I have a battery of questions about it, but they're for another thread at some stage.
I figure it's easier to keep everything in one machine with a large drive for backups and stuff I don't need to access all the time. I gather video editing is faster with an external drive too as the files are not competing with the OS on the main drive.
Portability is very important to me. Up to recently I would have said that the combo is ideal, but if you need to travel a lot, it is handier not to have to worry about syncing and such. It won't look as neat on the desk, but that's not so important to me, and there are creative ways to make it look nice.