I was able to register this morning with my Premier Rewards Gold card. I don't carry my Blue with me so I'll have to try that later.

However, I wasn't able to get into the iTunes store, probably because today is the first day that the Verizon iPhone is on sale in B&M stores.

That's also the likely reason for this promo to start today. They want to get as many people as possible to register their AMEX card as the default payment option.