There's a thread I started too:
https://forums.macrumors.com/threads/111677/
It seems to be a somewhat common issue. I can trigger it on any apple laptop I use given a week or so. Our iBooks have third party ram, but the powerbook's ram is from Apple (dev. discount, no I didn't pay their rediculous ram prices).
Apple claims to have fixed the issue in 10.3.8, but it is still there.
Hopefully with enough people talking about it, something will be done.
If you want to check if the computer is still on and just has a black screen (my problem) then you can try SSHing in or connecting to a file share. My computers all work fine in this state - except you can't see anything!
Only solution I've found is a reboot. It really sucks because this is the only thing I ever end up rebooting for (besides an OS update).
Try turning off the option to spin down the hard drives when possible. I've found that helps, although it isn't a sure thing.