I love the idea. I have tried to use "CardStar", a great app that holds all your store rewards cards for companies such as Duane Reade and BestBuy. The only issue is some store scanners cannot read/scan the bar code from the iPhone screen, but they can enter the numbers. Saves the effort of carrying around a dozen cards. NFC would be a better implementation.
Question about Starbucks. I set up the account, and entered my CC info. Yet it seems you have to buy a Starbucks gift card and enter the info in order to use it. Why not charge the CC# entered into the application instead of having to add a third step, or am I missing something?
Do you need to confirm the payment on your iPhone? What is the radius of NFC?
"Minority Report"
Since the iPhone NFC isn't real yet or known publicly hard to say. I would imagine you would have to confirm on the iPhone. As I see them used now with NFC credit cards, you touch your card to the credit card reader and then have to confirm your transaction.
The radius of NFC is listed around 4 feet. However, I think for current systems like credit cards it is much less. Not sure if this is due to the small size of a credit card and how far it can broadcast or a limit set up for NFC payments. Certainly an issue if used in an iPhone because I wouldn't want to be standing in line and get prompted (note not automatically charged) 2 or 3 times while waiting for the people in front of me to check out.
NFC does rely on "handshakes" so the iPhone is only one part of this equation. The NFC "reader" used in a check out line could be limited (I think) to only request information in a smaller radius (like 1 foot or 6 inches), where as advertising signs might reach out to 4 feet.
Certainly privacy is an issue and if Apple does it right where you can turn on/off NFC, maybe even limit what it is used for (payments only/ads only/personal only) along with prompts it could bring about a change in NFC usage. It has been around for years now but isn't very widely used.