Not many details are available on how the consumer will ultimately pay for the scanned items, either using the iPhone or via cash, check, or credit card at the counter on the way out. In the test, the data is sent from the iPhone to the cash register at the self checkout line, and then the customer pays in traditional methods.
The ideal situation is to scan and bag, and then be able to indicate on the app that you are finished and payment is automatically taken off of a stored credit card in the app. This works really slick in Apple stores.
This type of technology is going to become increasingly popular as time goes on, and I am looking forward to the day when I do not have to carry a wallet.