I was looking around to see if anyone started a thread on this already. I received my new BTO iMac 21.5, i5, Fusion, 16GB yesterday and after configuring it alittle bit, I went ahead and installed Civilization V + expansion and spend a few hours playing it.
Afterwards I felt all around my iMac and I could really not feel any substantial heat. I would say like the iPad feels when using it. The only area which felt warmer (I cannot call it hot) was the area around the FAN exit.
I have used my 2007 model iMac alot for gaming and the upper left part of my screen became white'ish because of the heat I think and it became so hot I needed SMC fan control.
Nothing like that on this new one but one night of Civilization 5 isn't ofcourse a very representative test... but so far so (very very) good
PS: I was also trying Diablo III for a short while with max settings and was getting around 30-40 FPS