^^^ what vansmith said.
This is most likely yet another case of a user misunderstanding how the iMac cools itself (and, to be fair, it is quite different and far more clever than most other non-Apple computers).
The iMac uses convection cooling in part, to keep fan activity to a minimum so that the machine can run dramatically quieter. For those who slept through high-school science (raises hand), this means that the design is based on the principle that heat rises. Cool air is drawn in from the underside of the iMac (using gently-run fans), and passes over the hot components. The air is then naturally vented out of the slot near the top of the back of the iMac, which (if a user felt it) would *naturally* feel quite warm, but this is in fact the hot air *escaping* and thus exactly as Apple designed it (I hear they're actually pretty sharp on this "design" business, and don't need the kind of micro-management some PCs do).
Bottom line: what vansmith said. Computers can handle temperatures far higher than humans can, and so long as the heat is being vented properly it is working as designed. In addition, should something like a fan failure occur, there is a failsafe already in place. In short, users need not concern themselves with this.