This Applecare document may fix the battery problem I (and others) have run into with iBooks and PowerBooks. Apparently the power manager circuit can get cranky and needs resetting. I'm about to try it. The downside is: my iBook (which I use a lot lately) will loose it's precious 73 day uptime.
Followup: The fix seems to have worked. Considering this lovely machine is about two years old, it's probably time to get a new battery anyways. Without the problem (where the battery goes to 0% with little or no warning), I still get an estimated hour - hour and ten minutes use. That's not too bad for a two year old battery, I think. The time is made worse by the machine having sat around unused for a few months, plugged in and always charged. The quest for the perfect battery is far from over...
Followup 2: Up for 47 minutes now without AC Power. I think my problem's definitely fixed, which means I can take my iBook out on the balcony again without fear!