I don't want to repeat all the wise words from everyone here (particularly the link to the "Overheating" sticky), but I will just remind anyone reading this that iMacs (in particular) use CONVECTION COOLING to help keep the fan use/noise to a minimum. This means the design *deliberately* relies on the principle of warm air rising. Cool air is drawn in from the bottom of the machine, passes over the components and gets warm, and rises out the back vent. IOW, the back vent is SUPPOSED to be warm (sometimes very warm) to the touch. That's why it's on the back, keep yer mitts off of it!

In short, Apple thought about this and its not really anything you need to worry about.