I think people are misunderstanding. My bet is that Apple will not only support AMEX, Mastercard and Visa, but Store credit cards as well. Maybe I'll coin it the iWallet.
Stores LOVE having credit cards available to their customers....they make money on them like the big 3.
Apple will want every retailer to accept NFC payments with whatever credit cards people choose, not just Amex Mastercard and Visa. Users will have a digital wallet, fingerprint scan while your approaching the till, select what card to use and BAM...payment is made with whatever card you want....Nordstrom is just the first of many. No wallet required.