I was majorly annoyed when this was happening so I when to check the power management settings (System -> Preferences -> Power Management) and made sure that "Dim Screen When Idle" was unchecked for both AC and battery settings. Darn both of those options were unchecked and I just lived with having to move the mouse for a couple of weeks.
What you need to check is your Screensaver settings (System -> Preferences -> Screensaver). By defualt, the screeensaver is "Blank Screen" and the timeout for "idle" is rather low. So uncheck "Activate screensaver when computer is idle" or increase the slider for "Regard computer as idle after...". This solved my problems and I'm posting this so I don't forget.