Try an experiment together:
With your Powerbook fully charged, disconnect from ac adapter and try working in say a dimmly lit living room starting at full back light illumination. Have the energy saver set to automatic.
Then decrease a notch at a time going a minute or so before making the next decrease. Watch how the remaining time readjusts and recalculates as you do this. I have been working for over an hour in my shorts and the case is hardly lukewarm on my thighs(I swear) Often I can work in near darkness at one notch above blackout. One evening, just typing I managed a true timed 3 hours and 43 minutes typing and surfing.
Try this on your Powerbook and share/report your battery times and heat experiences with all of us noting Model and revision of your Powerbook and which version of OSX.
Does your Powerbook sizzle?
Here is a September 2003 17"1.33 (currently 512mb) using OSX.2.8 running acceptably lukewarm.
Also, does anyone think adding more memory adds more heat?