Yes that's something that's greatly improved over the old iMacs. I had a 2010 imac that would get hot to the touch just browsing the web watching YouTube etc.
This baby stays cool even when playing games like starcraft, diablo 3, and a few other games I tried.
I, too, have noticed this. And seriously, it's about time; even my Early 2006 20" iMac would be noticably warm after heavy use. And it only got worse from there.
The iMac is getting thinner every 2-3 years. I wish it could house a GPU that isn't mobile but it will not sadly till desktop graphics shrink. To like 1/3 of it's origin. 🙁
Digitalversus.com review of the 21.5-inch and 27-inch models includes thermal images of the front and rear of the iMac using a Fluke Ti25 heat sensitive camera.