By opening Battery.menu in the folder System -> Library -> CoreServices -> Menu Extras, you will get a new menu indicating battery level and remaining time if you are running off your battery. I haven't had the nerve yet to see if my Mac will automatically shutdown if the battery level goes too low yet, but I don't see why it shouldn't (anyone want to try?).
This will only work if your Mac is running 10.3 and is plugged in to a compatible USB UPS. Jaguar seems to have some support for UPSes, but not in the Battery menu extra.
[robg adds: I don't have a UPS with which to test this one ... can anyone else confirm this behavior?]

