.. he could transfer some or all of the value of the unredeemed coins into an address controlled by him. It would take a while - maybe years - for anyone to even notice, you'd have to try and redeem it yourself. And even if you did notice, how could you prove it was him and not someone else who had access to your coin in the intervening time? You couldn't even prove that you hadn't already redeemed it yourself. The whole concept is taking everything that is good and secure about bitcoin and shitting all over it.
That said, great idea for a novelty memento, I'm sure they will sell like hot cakes for that reason alone
I have PGP-signed all of the bitcoin addresses I have used so far in the coin project and thrown the whole thing in a pastebin (link in my original Casascius Bitcoin thread). If a large number of my coins suddenly got spent all at the same time - especially if those physical coins are still intact - it would be pretty darn easy and bulletproof to prove it had to be me. That and the fact that I go by my real name and address and am hardly insolvent... you guys have pretty sound recourse against me if I were to pull crap like that.