You could even have online activation to prevent people from stealing cards with value on them. You create a MyBitcoin.com account with a secret password, deposit the bitcoins in the account, print the user name and a different password on the card. When you receive payment for the card, you simply change the password to the password that's on the card. Ideally, you would actually create your own bitcoin redemption website instead of using MyBitcoin.com since they might not appreciate all the disposable accounts. If you don't have a problem with fractional reserve principles, you could print a lot more cards than you have bitcoins and deposit the bitcoins from your reserve whenever a card is sold. You use the proceeds from the sold cards to purchase more bitcoins. Spikes in the price of bitcoins could cause a problem, but if you somehow made the price of the cards dynamically dependent on the current exchange rate then it wouldn't be a problem.
