On PI, I agree that there is a qualitative difference between a cashier and a machine if you assume, as most people do, that the cashier will not remember private details. This can be fixed with industry-wide agreements to not store such data, in the same way as merchants don't store your full credit card details. You could even allow people to bypass the check if a valid identity is registered against their store-issued fidelity card.

Honestly, the only hard problem is localization. At the moment you can sell more or less the same automated till to UK supermarkets, US supermarkets, or EU supermarkets; but document-checking code will have to be heavily localized. Still, EU driving licenses are already similar on the entire continent, and that's quite a critical mass one could hit. I would be surprised if manufacturers weren't already developing this feature.

Although I'm way beyond the point of caring about demographic profiling, I still don't want my home address to be available to everybody dealing with a simple purchase (although this perspective is probably greatly atypical on account of being a survivor of CSA).