By drilling it out you are then using a hex bit to unscrew it, correct?
I've got to do something similar with a hex nut - I was thinking of putting a little JB Weld on the end of an allen key and letting it set... anyone else done that?
For future reference cutting a slot in the screw head with a Dremel has always worked for me.
I rounded the screws in the bottom of a Macbook Air once and attacked them with a drill! I ended up slipping with the drill and going right through the entire laptop and out the other side.. Whoops!