my son has just done exactly the same thing with his black ops game,only been playing it for a week or two,now it has a deep gouge in a perfect circle but he never even moved the x-box(so he tells me )
Well i knocked mine from standing to flat with the disc spinning.
Ive done it before with GTA aswell and no amount of buffing will get it out
Its currently £29.99 brand new in game so im willing to pay £20 on here for a second hand copy