If you look at the benchmarks the current low-end 4 core MP beats out (barely, but still) the top-end iMac i7. On top of that the iMac has a glossy screen which isn't useful in the pro world. The Xeon chips in the current MPs are the Xeon version of the i7, which of course was release months ahead of the i7 iMac.
I would expect (and counting on) new "Gulftown" based MPs around March/April. I also expect a new non-glossy 27" monitor to replace the current 30".
+ 1 for that - especially if they have sorted out the 27" iMac screen issues.
I don't think they'll be releasing a 27" non glossy to replace the 30". For one there's no way they'll go down in physical size. They're more likely to release a 32" screen with something like 2844x1600. Chances are it'll also be glossy - not that I think that's particularly a problem even in the pro market. 2010 glossy ≠ 2003 glossy. I work quite happily on my glossy MBP and I virtually never notice reflections because it is so much brighter.
The fact that the 27" iMac was 'only' 27" was ultimately what stopped me from replacing my setup with an iMac. I had a 2006 Mac Pro and a 30" ACD. While I could have maybe lived with the slight drop in resolution, I simply didn't want to lose those 3" of screen space.