mwh: Well, from the documentation that came with the tibook (I had to hunt that one down :-), it says "If your battery runs low while you are working, you can put your PowerBook to sleep and replace the battery with a charged one. The internal backup battery provides enough power to maintain the contents of RAM for one minute while you change the battery."
So there you go ;-)

(Hi, rest of advogato! I've started fostering some open source OS X apps on freshmeat.net for people interested. So far there's Fire.app, and hopefully more to come.)