Depends on what you are doing any laptop would heat up once your working on something. This shoulden't harm your mac unless the fan is working over time and you start noticing extreme temperatures and rapid shut downs. It is not true that macs dont heat up as its almost imposible to not get heat form your laptop. Extreme temps are when you actually cant hold your laptop in your lap and must set it down. Just to be safe run a diognostics test using your original Diognostics CD that came with your mac. If you do not have your CD Download the image from Apple Support. If it is some thing with the hardware than most likly its your fans/heat sink. Also the easiest fix could be you need more thermal paste on your CPU as your thermal paste got old and dried out. Good Luck.