My 2 pence worth
I was looking to replace my ageing iMac last year with the latest iMac.
Call me paranoid, but I was put off a bit with some of the negative posts about the new 27" version.
As I travel back and forth to Holland for work, I decided to go for the 13" MBP + 24" LED cinema display option, and wait for a future generation of the iMac.
The MBP is the 2.53GHz, 4GB version, mid 2009. I do a lot of Photoshopping, a fair bit of web design, regular video editing, and use both the Microsoft and Apple Office suites. I don't feel my setup to be lacking in grunt unless I'm working on some huge multi-layered image files in Photoshop. If your work doesn't exceed those demands, the latest MBP should be fine. I find 13"to be a perfect size for portability, and the 24" monitor is a joy. I also added a full size keyboard for home use and Magic Mouse for travelling (I use a graphics tablet at home).
The only other advice I could give is to go to an Apple store if you have one nearby, and compare the iMac against the MBP + display.