Starbucks updated their iOS app to support Passbook. Passbook is a new feature for iOS 6 that consolidates retail point of sale cards into a single interface. These cards may be used for reward programs or store credit, and the Starbucks app supports both. Users will need the retailer's app installed, and then the app will ask if users want to place a card in the Passbook.
In addition to managing the virtual cards, Passbook will prompt the users when they reach a favorite location. The prompt appears in the iPhone's lock screen so they can quickly jump to the virtual card.
The Starbucks app is one of the early adopters fully utilize the Passbook features. App Shopper has a good index of apps supporting Passport.