It just turns off.
There's a clue, in that the battery indicator will change from black (some charge) to red (really not very much), but that's not all that noticable. And then suddenly you're typing away, and the screen goes black.
I'm sure there used to be a setting that said, "g'wan, be nice, please give me a moment's grace to shut everything down," but I can't find it under Energy Saver. Grrr.