By CPU I imagine you mean the proc die itself and not the ambient case temperature.
My PIII 800 (non-OCed) tends to run from 70 to 90 degrees F, depending on the temperature of the room (we haven't air conditioning). A PIII 500 (also non-OCed) runs in a fully packed, 5-bay case and levels around 100 to 115 degrees F. Of course, it's also in a poorly ventilated area. Both computers are equipped with large case fans, two power supply exhaust fans, and a Gold Orb proc cooler.
As you can see, there doesn't tend to be an "average" temp, seeing as how fans, clock speed, die size, space inside one's case, and ambient room temperature can make a sizable difference.