I think I'm starting to see similar problems with the mouse that
others are reporting.  I was running the lastest xorg compiled from
scratch.

The machine in question has one of those logitech USB mice with a
translucent red base.  During normal operation this base section glows
red and is quite noticeable.  Yesterday the mouse just turned itself
off.  I didn't even realize that the led power was under software
control.  

As others have noted, unpluging and replugging the mouse got it up and
running again.

-wolfgang