Matt from the UCC details how to enable safe sleep (or ‘hibernate’, if you’re from the Windows world) on PowerBooks that don’t normally support it. Be warned, it’s not for the squeamish as it requires some Open Firmware hackery, but scripts are provided.
Safe sleep basically writes the contents of your Mac’s RAM onto the hard drive when it sleeps, so even if the battery goes totally kaput (or, for example is removed), your Mac should theoreticaly start back up where it left off when you apply power and turn it on again.