Apple is creating an app in iOS 6 that's the digital equivalent of stringing frequent-shopper cards on a keychain. Passbook allows you to store these sorts of cards, airline passes, movie tickets and more. The passes will appear on the lock screen. Swipe the notification, and a QR code will appear. Other businesses glimpsed include Starbucks, Amtrak, the Apple Store -- even a Giants baseball ticket!
As far as airlines go, it looks like just United Airlines is onboard for now. Multiple boarding passes are grouped together. It's integrated with the notifications center -- being near a favorite Starbucks will alert you. You also can get gate changes and more information with the airline tickets.
This sort of usability will be fantastic, and I would love to see more businesses and libraries get on board with this. It will definitely reduce the amount of time spent hunting in your wallet for a particular card.