I thought about this and it would be a great idea. Today's solutions typically involve scanning a QR code and the Bitcoin wallet sends the transaction over the Internet. But when you have poor signal, this does not work well.

Instead, a Bitcoin wallet could send the transaction (a few hundred bytes) over NFC. This gives an extra advantage: the point-of-sale would broadcast the transaction to hundreds of Bitcoin p2p nodes itself, hence reducing the risk of accepting a zero-confirmation transaction!

Would be cool to be able to tap to pay instead of scan to pay.