I just happened to be at Defcon yesterday when Stits and Datagram managed to peel the holo foil off of a Casascius coin and replace it with basically no real damage. They think with a little refinement of technique they could pull it off with no visible damage at all. Caldwell is in talks with them trying to improve security, but for now you should all be weary of second-hand Casascius coins.

http://codinginmysleep.com/casascius-physical-bitcoins-cracked-at-defcon/