from the looks of it it looks like it functions this way:
1) Install an app that uses passbook (I tested with walgreens)
2) Register for something that would allow you to add a card to passbook (I just registered for the rewards card)
3) When it asks to add to passbook, choose it.
4) When you open passbook now, it has your card there getting updates from whoever created it.
It kinda stinks you need the app to use the passbook features, i was kinda hoping to just have passbook without any of the random apps. But still hopefully that will help people get started.
Also i dont see where starbucks has updated their app to support it quite yet
what about for...fandango for example? this should be supported now, however it doesn't actually show up in passbook when you open the app.