Swatch are in for a good shout
They've got plenty of experience with making 'near-field' watches reliable for years. They've had a 'programmable pass' watch for at least 20 years that's compatible with things like Ski-lift system in resorts.
Shoving in an NFC chip instead of a proprietary one, and programming an associated bit of memory with bank account information would be a p-o-p (essentially taking the chip out of the credit card and making it editable). It's an ideal use case scenario for something like Oyster readers too - no need for a tinfoil wallet - with only 1 NFC 'device' near the reader there's no chance of duplicate-payments. Security is no doubt doable.
Come to think of it.... why ISN'T there an Oyster (S)watch??
(or is there? - swift google : http://www.trustedreviews.com/news/Swatch-In-Talks-To-Make-TFL-Oyster-Watch : I bet the 'talks broke down')