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.
Normal Operating Temperatures
Most people will ask me what my Mac's normal operating temperature is, and I will answer them: "The normal operating temperature doesn't really matter much." Why? Because it will differ from situation to situation. Your laptop's temperature will change depending on usage, air flow, the surface the laptop is on, and many other environmental factors. I will go through ideal operating conditions for low temperatures:
1: Surface: What your laptop is resting on is probably most important factor for controlling its temperature. If you have your laptop on a carpeted or soft surface that could absorb a lot of heat, your laptop's temperature will be dramatically higher, for there is nowhere for the hot air on the underside of your laptop to go.
The best solution is to either always use your laptop on a solid surface (your legs are okay, too, because they leave room for air to flow underneath), or to purchase a laptop stand or fan pad so there is airflow underneath your laptop.
If you use your laptop on the ground or in your bed often, perhaps a solid board (even one with a pillow for comfort on the underside) should be used under your laptop. This will allow your laptop to let air flow underneath.
2: Air Temperature: Another controlling factor is the ambient air temperature where you are using your laptop. I would not expect your laptop to be any chillier than it's surroundings, so I would recommend using it in a place where it's 80° F or less. This will allow your laptop to cool down instead of heat up if you let it rest.
3: Air Flow: You should make sure there is at least some sort of breeze wherever you use your laptop. It need not be continuous, but the area you use your laptop in should have some airflow. Otherwise (if the air is stagnant), the hot air around your laptop will have nowhere to go.
4: Usage: If you are typing a paper, doing some light Internet browsing, or checking email, your laptop's processors won't be producing much heat, so you need not worry too much about operating temperatures. However, if you're rendering video files, recording and mixing video and audio, or working on large files with fancy programs such as Photoshop, you should try to keep your laptop as cool as possible!
5: Cleanliness: Believe it or not, the amount of dirt and dust in your environment is very important to maintaining a proper temperature. The fans in your laptop need to be clean to work with optimum efficiency, and extra dust and dirt in air vents can block airflow. Keep your environment clean of as much dirt and debris as possible. This means dusting your desk, vacuuming your carpets, and making sure your cooling system's filter(s) are clean.
What I Do
For my Mac, I set it on either a Targus Notebook Stand (no fans) or a Griffin Elevator (depending on where I am), which gives adequate air flow around the bottom of the laptop. I usually have a fan blowing in my room and office to keep constant circulation. I clean the fan, my laptop, my desk, and the rest of my room monthly (dusting, disinfecting, vacuuming, etc.). I also have a temperature monitor constantly running to let me know if something is making my computer particularly hot. I try to keep my room at a constant 68-72° F.
Conclusion
Basically, it's up to you how much you want to do to keep your computer running clean, mean, lean, and cool. I recommend you try to keep everything as cool as possible, as this will save your laptop from any possible overheating damage.
*This seems to more often be the case with MacBook Pro and iBook laptops, especially older generations. The newer Pro laptops are getting a little cooler, thankfully.