I've been waiting for the new model for a few months now and in the meantime set up my 13" 2010 MBP as a desktop to convince my wife to change from windows to osx. This is OK as an interim solution but is really too slow for a desktop and I also need the MBP for work at times. I won't buy the current model Imac as I got caught out by the 2011 MBP refresh and now how an 18 month old machine that can't even run all the features of the current OS (i.e. airplay mirroring)
Out of frustration, I decided to try to Hackintosh my 3.5 year old 1st gen Core i7 920 windows PC with GTX 260 GPU.
Now running mountain lion with a Geekbench score of 9200, which is ~2.5x my MBP score.
I'll still buy a new 2012 Imac , probably the 21.5" higher spec model, when it comes out as I really want to get rid of the large PC tower, but I'm not in any rush now.