Sainsbury's in the UK has been providing a service for a while now where you walk in, pick up a bar code scanner and scan your own goods as you put them in your trolley. It works really well and you save loads of time and queueing at the end of your shop. Using your mobile would take it one step further. The worry I'd have is that you scan all you stuff, get to the checkout click the finish button and you get that little spinning circle and your device freezes because you installed some fancy finger interface Beta! 