Looks like it's the motherboard. Yesterday I powered it down, removed the battery, and started it back up. By this morning it had again shut itself down due to overheating. Rats.

I actually meant to say this earlier - your initial symptoms sounded like what happened to 2 straight Toshibas that I owned in the 2004-2006 period. Forced shutdowns, then blue screens of death, then the motherboard fried. I used both laptops heavily for about 15 months (and they were great), but then they died.

FWIW, I've owned an Asus laptop for a year now, and it (knock on wood) has been perfect.