It all depends on your work habits (desktop or "on the road").
Back in 2007 I faced the same question. I was a life-long Windows user, and decided to switch... I went with the 24" iMac - I wasn't sure mac would perform well in the mobile scenarios in my line of work, so I decided for the desktop and kept my Toshiba laptop. As I got familiar with OSX and mac, I just couldn't use windows anymore. Two months ago I got myself a MBPro.
I love this setting: I have a huge screen to work at home and a laptop that syncs all my settings and files when I travel, without having to plug/unplug all connections every time I leave (I have prayed that apple someday has a decent docking station for its notebooks). But if I had to choose again, I'd get the MacBook Pro first: I could've ditched the Windows laptop earlier... and could've saved time and money with just one computer: you can always connect a 24" display to the MBP - the new one seems great.
Choosing the Macbook Pro + 24" display may be a more expensive solution in the short term, but saves you money once you realize how great is OSX and that you need to take it with you.
So it's just a matter of your work routine and the advantage of mobility. In terms of performance - it's yet to be seen if the iMac will get any big updates - but considering the different architectures, I would guess there won't be any huge differences you can notice in normal (non-pro) use.
hope this helps. cheers,
EDIT: I realize you're considering a plain MacBook vs. the iMac - the new MB lacks Firewire, and even if it has the new GPU and memory, it doesn't has the same power as a MBPro or 3 Ghz iMac. I guess the GPU and RAM cache on the iMac will be upgraded shortly.
Being this the case, and IF portability is no issue for you, I'd go with the iMac: better performace, ports, etc. You can always bump the RAM to 4GB in both for less than $100.