These are skimmed much more easily than credit or debit cards - all you need is a camera, not even physical access. There is also the issue of change. Wasted paper, too. Any smartphone can display QR codes on demand, only when needed. Perhaps a nice chimera would be an e-paper note. Sounds too much like the "Bitcoin Card"? It does, but I think Casascius is more than qualified to look into similar solutions and to innovate.
If you have a smartphone, why go through the trouble of displaying a private key to begin with, instead of simply creating a Bitcoin transaction yourself and sending the funds to the vendor's address?I like the paper throwaway bills (and these aren't the first ones we've seen). They would work well for the purpose they are made for. I think it was mentioned before in a different thread that the likelihood of a scammer being able to create a transaction to skim the funds even just 10 seconds after the vendor created their transaction would be less than 1%, assuming the scammer didn't hold a significant amount of hashing power. Remember that the standard client will reject second transactions if a first has already been broadcast claiming the funds. And if the scammer DID hold a significant amount of hashing power, what would they be doing with trying to scam 1 BTC here and there in in-person transactions?
