I remember reading somewhere that a feature of OS X was that you could schedule shutdowns. I just got my Powerbook (still Jaguar) but I can't find an option for this anywhere. The closest thing is timing the computer to sleep under energy saver.
that might be talking about Panther. in the new System Preferences you can set when you comp starts up and shuts down. i don't think you can do that with Jaguar, but there might be a app that can schedule shutdowns.