spontaneous logout

[David]

I'm still running hoary, which I tend to leave running 24/7, with various 
windows open.. mostly terminal and browser.

Every so often it logs out spontaneously for no obvious reason. The 
machine does not shut down. The login screen is displayed. 

It's happened about 4 times. No damage seems to be done except for the 
loss of the open windows. I wonder if any one else has experienced this or 
knows why it happens.

David