I was going to pull out my hard drive yesterday, and somehow, someway turned the screw driver but not the screw. All you need is a drill and a drill bit of some small size.
Simply put the bit straight down on the screw, and go to town. Go slow at first, and as you speed up move the drill in small circles... and after a while the head will fly off.
The thing that sucks about this is you cant put another screw in, but at least you got the drive out, eh?