A question oft asked on the Apple Discussion boards (and other online computing forums) is: "My laptop seems really hot on the bottom* - is this safe?" This page will attempt to answer this and many other questions about iBook temperature concerns.
Why is it important to control (to a certain extent) your computer's temperature? Because a computer is like a car: if it's too cold, it won't run, and if it's too hot, it will overheat. As with all physical objects, a computer must obey the laws of physics, and when the temperature is too high or too low, things inside the computer won't work well. The optimal operating temperatures for your specific computer should be listed in the computer's manual.
Temperature Monitoring Software
There are many free software programs to help you monitor your Mac's temperature (whatever model it may be). My favorite is Temperature Monitor (free). Temperature Monitor allows you to see all temperature sensors in windows, the Dock, the menubar, or your Dashboard.