There are many existing restaurant apps[1] that do this now (interface with POS, pay with CC), it isn't a big leap to think that it would be easier for me to order a burrito with my iPhone by clicking 'Pay with my Apple Account' versus having to enter a CC number.

[1] - I'm most familiar with Chipotle's http://itunes.apple.com/us/app/chipotle-ordering/id327228455... - which allows for all sorts of custom orders.