You need to follow this link in Safari on your iOS device: http://aexp.co/vn0
I registered one card, and it worked, but I wondered about my other cards. I asked the helpful folks at
9/28/12 update: I discovered today that "Cards issued by third-party financial institutions are not currently eligible for this program." That means my Macy's and Citi AAdvantage AMEX cards will have to be lonely until Passbook can handle them.