I've seen suggested a few times that stores offer customers promotional rebates of bitbill (or some other form of a physical private key).
From the perspective of a merchant, the likely return on investment seems small. Many customers would simply disregard and promptly dispose of their bitbills.
So I suggest printing a bitbill with an expiration date. The merchant would retain the private key. After a period of time, if the coins have not been spent, they could be reclaimed by the merchant. The financial cost of the promotion should be greatly reduced.
Except for a little detail. If the customer simply imports the key into the bitcoin-qt wallet, there is no transaction on the blockchain. And since the Bitcoin client doesn't let the inputs be chosen, the customer can't really force that bitcoin to be spent unless the client happens to choose it as an input for a subsequent transaction.
The other thing is the merchant has a liability then as well. There is the potential that the customer would even add to that address and the merchant controlling the private key yet could make the merchant be on the hook if somehow that address wasn't kept securely. If there were a security breach, not just would the rebate be lost but any other funds added by the customer as well.
A decent solution would be to put a recommendation onto the physical bitcoin a message stating to redeem only on Mt. Gox (which instantly sweeps any amount on the coin to a Mt. Gox address). Of course, a technical user can still import it wherever they want but the non-technical user would be protected.
I wonder if Instawallet URLs are preferable.
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http;//www.instawallet.org 
