Mac Minis are really power houses for their size and generate an equally high amount of heat. The aluminum enclosure actually acts as a rather effective heat sync (also why iPhones are made from aluminum and get pretty hot). The Mini has been tested to with stand up to temperatures of 200 Deg F! And at that temp the mini would reduce its own power to the cpu to help with heat dissipation. A few tips:
1) don't cover it with things like books, external drives etc as this can generate its own heat and will resist letting the mac mini vent.
2) don't surround it with the same as #1. The mini actually has vents at its foot and rear to allow for air flow. Keeping these open will keep your mac humming nicely!
My 2.3GHz i7 Mac mini gets warm only if your doing processor intensive tasks. But the cooling system can handle it and it never gets ridiculous to the point that it overheats or anything. Under normal usage it doesn't even get warm.