I wonder about the receipt step -- are you generating a PDF on device
and sending it via NFC? This is something that could be supported by the
BIP70 payment protocol. We should try to avoid the second tap, its not
intuitive.

Together, the signed PaymentRequest and the transactions in the block chain should act like a receipt: it's proof you requested payment in a certain way, and I satisfied that payment. So it's proof of payment and the memo field can describe what I bought.

Contactless cards? Last I was to London, the Oyster card was already
contactless. Have there ever been magnet-strip-based Oyster cards?

He means, contactless credit cards can be used too. No need to enroll for Oyster specifically. I guess in the long run Oyster and its equivalents in other cities (octopus etc) will be phased out.