I bought Snow Leopard on the first day it came out, and have been having this problem throughout. I have tried all of the solutions recommended on this board. NONE OF THEM WORK.
The one thing I have found is that the problem absolutely never happens when I am traveling (and I have been out of town about 50% of the time since I upgraded). On the other hand, as soon as I return home, it happens pretty much within the next 48 hours. This leads me to believe that Time Machine is part of the problem -- my guess is that it is the simultaneous activation of Time Machine, Mail.app, and iPhone, checking a POP account, that causes the problem.
APPLE, IF YOU ARE OUT THERE LISTENING, THIS IS AN OUTRAGE. After all the potshots you took at Vista, I am having disastrous problems with Snow Leopard. Not only this, but I can't seem to mount my Time Capsule hard disk anymore (though Time Machine, strangely, works fine). Another external hard disk was ruined. Come on people.