I had the old aluminium top-end, maxed out iMac (24" screen, 2,8GHz Core 2 Duo Extreme, 4GB of RAM, 500GB hard drive). On the 14th of October, when the new MacBooks were introduced, I ordered the top-end model (2,4GHz Core 2 Duo, upped the RAM myself to 4GB, 250GB hard drive, but will be replacing that with a Seagate Momentus 7200.4 500GB drive once it's out). I got the MacBook as a companion machine to the iMac, as I always thought the iMac would be and stay my main machine.
Well, to make a long story short... I sold the iMac. The reality was that I hardly used it anymore. I've always been a desktop guy, but it seemed to be like eating fish. I never ate it because I thought I wouldn't like it. Well, once I started eating it, I couldn't get enough. So now I'm as much a mobile convert as I'm a fish lover. I'll never go desktop again. The MacBook does everything my iMac could do. Sure, it might be just a tad less powerful, but that is so compensated by the fact that I can take it with me to wherever I go, it's not even funny.
So my advice would be to go with a MacBook. If you really can't do without a large screen, you could always get an external screen with it. Ideal situation, if you ask me. My girlfriend digs the MacBook better than the iMac, too, as we could get rid of the ugly desk and (even uglier) desk chair.