I just discovered an interesting . . . bug? . . . feature? . . . let's call it a "trait" of Windows XP. If you want it to go into hibernate, it'll ask for confirmation. As far as I can tell, while it's waiting for you to say "yes, I really do want you to hibernate," it stops watching the clock for power save mode. So what's the practical upshot? Suppose, hypothetically speaking, you were in a meeting. At the end of the meeting, you hit the key sequence to hibernate (Fn-F4 on my machine, your mileage may vary.) You forget to click on the little box that asks "do you really want me to hibernate". You then close the lid and slip the machine into your briefcase. When you get home, you find a rather warm briefcase and a machine still patiently awaiting confirmation, even though the trip home was long enough that a machine on battery power should have suspended itself.
I wonder how long a machine will stay in this mode. For example, would it run the battery all the way down, losing whatever work's in RAM? I'm afraid to try it with this machine right now because it sees constant use and breaking would be a Bad Thing, but maybe that's an experiment for the future.
 You can disable this confirmation box, but I haven't because I wouldn't want the machine to accidentally hibernate in, say, the middle of taking notes. Hypothetically, of course.