It's a given that they will run very hot under Windows in Bootcamp, Mac Pro excepted.
I have the 27" iMac Core i7 860 at work and under windows unless I use SMCFancontrol, it will run so hot to the point of stability issues and red boxes on the screen. This was reproduced on multiple machines of the same spec (4 of them).
At work, I've accepted that the only practical way to run Windows is through VMWare otherwise I'll have to put up with noisy fans in the iMac. A colleague has a MBP which also runs very hot under Windows using Bootcamp.