If you overtightened the bolt you stand a good chance of getting it out.
 If there's a stub protruding try just gripping it with pliers; once
the head snapped off it should have relieved most of the tension causing
it to be hard to turn.  If that doesn't work, take a small piece of
hardened steel shaft, grind a point in the end of it, and try to "walk"
the stub out with a hammer (get it to rotate counterclockwise until you
have enough sticking out to grip)

If neither of those ideas work, THEN break out the drill bits.  A good,
sharp drill bit is your friend here, as is lots of oil and a friend to
drizzle it on the bit to keep it cool.

good luck,

nate