The easiest way to fix this problem is to simply file the tip of your pentelope screwdriver down until the grooves are big enough to catch on whats left of the stripped screw, I just had this problem with my iPhone 4s and it took only minutes to get back on track!
Another perk of doing it this way is that your not making any physical change to the phone itself (e.g. using a Dremel or drill) ((WTF!)) so there is no risk of damaging/scratching your phone, you may render your screwdriver useless for future use, but at least you have removed the stripped screws without damaging anything else!!! :D