You people forget that the public key is visible. Therefore you cannot invalidate the card because you can look up the bitcoin address on blockexplorer.com to see if it has the funds in that the card claims.

Once someone decides to crack open the card, they'll be the first to spend them.

BitBills should include some kind of serial code id number which can be used to verify it on their website. That way you can verify that the card actually came from them. Maybe they could sell POS scanners that verify the tokens when swiped.

Very cool, you can even store more of your bitcoins in the bitbill address.
But I think paying directly with them isn't very secure.
Here's an attack:

1) I purchase a bitbill.
2) I put out the stick and scan the private key.
3) I put back the stick
4) I pay with the billbit. When the merchant verifies the bitbill, your server says. "Yes, the 20 btc are still there".
5) I use the private key to extract the bitcoins

Maybe the sticks are so special that make 3 impossible, but I don't think so.
Anyway, It will work great for gift cards and physical storing.
A bitcoin client that imports private keys from IQ-codes would be useful.

Very cool project ... can go in many tangents .. more heads to hydra ....

If we trust llama that he has destroyed any digital/electronic representation of the private key and the only place in the universe that it exists is inside that card then it is ready-made physical back-up as you would ever want to store large amounts of BTC. Just keep sending them to that public address on the card ... oh and remember not to give someone a BTC1 card that has BTC 10,000 loaded up on it!

From the FAQ;

"After each card has been produced and proven functional, we delete all records of the private key. This means that once the card leaves our hands, we can no longer access the associated bitcoins (be aware, this means we also can't help if you lose or destroy your card). Keypairs are generated on an offline computer that runs off of a flash drive which we will occasionally destroy in spectacular fashion."

I mean if he is doing all this to physically secure those private keys who can be bothered doing this themselves? It is another little niche market, providing secure, physical private keys that are not part of the electronic network and never have been (thus not possible to be subject to electronic tampering). Addresses associated with private keys secured like this are very useful as savings accounts. Good anonymity initially too.

Another idea: If you're going to continue creating small batches at a time, perhaps you should offer them via a second item auction (what eBay calls a "Dutch auction"). Set a minimum bid of face value, of course, or maybe even face value plus cost. It will maximize your profit and get these cards to the people who want them the most, meaning the people most likely to do something interesting with them.

I love the idea of buying a card and using it as a long-term "savings account". In fact, maybe BitBills should offer "savings cards" with no face value at all, for precisely that purpose! (Otherwise there's the small risk that the card will get mixed in with the rest of my money and I'll accidentally spend it for its face value.)

Yes kiba.

Wouldn't it be pretty obvious that someone put a sticker over the public key code?

Perfect security doesn't exist. You may as well say that Bitcoin isn't secure for trading, because someone could hack into your computer and replace your client with a counterfeit.

These cards are quite secure enough for trading, as long as you aren't foolish enough to accept huge amounts of them from a single untrusted payer without auditing them first. Right now I'd accept payment of ฿20 or so in Bitbills from a stranger without even checking the balance, if they looked right. For large amounts like ฿100, I'd probably redeem a few of the higher denomination cards, but not all of them.

What's wrong with that?

Can't we just do that ourselves? Just print out the private key, lock it up, and delete it on your PC.

Expiring notes will help new notes with better security get into circulation. A redeem or exchange by date on the bill itself will fairly keep the holder informed.

Good point.

I still don't think expiration dates are necessary or wanted.  I would rather have less currency in circulation, than have my bitcoins randomly expire because I forgot to redeem them by a certain date.  I HATE expiration dates on things.  If I get a gift card, I want it to be good for as long as I live.  Same with gift certificates.  Expiration dates are frustrating, obnoxious, and I would never purposefully buy something with them.  I'm too forgetful of a person to buy money that expires.