Recently, someone on a mailing list had terrible trouble with his iBook batteries draining very quickly (empty after 1 hour, 45 minutes max). He noticed the hard disk didn't spin down. Suspecting it was the OS X energy saver somehow not working, he searched days for a fix, but didn't find anything. System updates came and went, but still the problem persisted. Until finally someone dug up the following Apple Knowledge Base article:
PowerBook and iBook: Resetting Power Management Unit
I know it is perhaps not really OSX related, but I've seen other people struggling with this bug, thinking it
is OSX related, so I figured I could only help by spreading the word.