Yes, my 09 iMac does the same thing. I asked the local Apple Authorized Service about it. They told me the early aluminum models run hot. The power supply is located in that area.. basically the whole mac is a heat sink. He told me not to worry about it. The new iMacs run much cooler, at lest they feel cooler.
It depends what you are running as well. I run ITV Player (in UK) and the fans kick in and machine gets very hot when I do, the GPU must be located there as well?? My machine was fairly cool in comparison to my wife's but when I replaced the HD on her machine it ran far cooler, awkward but not difficult to replace. I've just replaced my HD, we'll see how that gets on. I checked out all the Mac i27 inches in local PC Worlds at that time and they all could fry an egg. Later versions seem better.
I have a 2007 iMac and had a 2011 iMac and both got pretty warm when I was was or am working them. My 2012 is a completely different story. For the most part it stays cool to the touch. The heat has never caused any issues though.