The back of the iMac is meant to get hot. It acts as a kind of heat sink. But if it's really too hot to touch that's not good. This is probably in the upper left corner of the back as you face the screen; it is where the power supply is located. Get Temperature Monitor (free) and post all the results, including the power supply temperature. It looks like you've been running something graphics intensive, perhaps a game or something with Flash content. This will always raise the temps.
http://www.bresink.com/osx/TemperatureMonitor.html
Actually, I don't see anything terrible in those temps, at least those you posted. But they might be lower. You might want to get smcFan Control.
http://www.eidac.de/
Not including the default, I have five fan settings increasing in increments of about 20% to around 2K. I give them different names such as "minimum," then "minimum plus" etc.
From experimenting, here's the way I use smcFan (you may find your own preferred method): I would start out at the lowest setting above the default. This will be approx. +200 rpm. See how the temps are looking using that setting. If needed go higher in steps. But you don't want to run the fans constantly at 2K. That may shorten their life.
When needed, I also run a small 6" clip-on fan mounted to the wall just behind my 21.5. It helps dissipate the case heat.
Make sure you keep the air intakes free of dust. I vacuum mine periodically, with the computer shut down.
