Preview at WWDC likely in June, followed by September launch.
Starbucks Adds Passbook Support to Its iPhone App
It's a near perfect use of Passbook; presumably why Starbucks is prominently featured in Apple's marketing. Passbook is designed to get users directly into gift cards or event tickets from the lock screen, using time and location information to discover when a user is near a Starbucks and put a link to the Starbucks gift card directly on the lock screen.
The Starbucks app is a free download on the App Store. [Direct Link]
Other apps using Passbook include Ticketmaster, Fandango, American Airlines, United Airlines, Lufthansa, Major League Baseball, Eventbrite, and McDonalds (as a limited test in France).