BTW: a QR code next to a cash register can be fixed i.e. printed on paper when using BIP70. The PoS would upload payment details to the server and the URL for that particular PoS unit would then serve it when the user scans the QR code. Alternatively, Andreas' work on Bluetooth may be more appropriate: the QR code can contain the BT MAC of the device and the payment request is downloaded that way. That's already implemented! I still feel that if a seller can scan a users phone, the users phone can certainly scan some rectangle that's physically near by the sales counter.

The other nice thing about that approach is the QRcode can also be an NFC tag i.e. have the tag behind it with a little icon in the middle of the QR code to indicate that touching works as well as scanning.

One project I keep wanting to play with is making these little NFC-QRcode hybrids and a simple PoS app to go with them. But no time, alas ....