You can store multiple credit cards in Apple Pay. One card is always set as your default credit card, but you can change the card you want to use before paying.
This was unclear during the announcement of Apple Pay, but it makes sense, especially as Apple Pay uses the multi-card interface of Passbook.
This gives me hope that iOS 9 — yes, I’m already thinking about 9 — might open up the NFC chip to more than just Apple Pay. Your phone could become your credit card, as well as your Myki or Opal card, your swipe card at work, the devices that unlocks your new NFC compatible Macbook Air.
But I’m getting ahead of myself. Let’s just get Apple Pay here in Australia, first.