Apple's Passbook-based iTunes Pass system for adding money to your iTunes account launched in Japan only days ago and now is available in the United States, too. The system lets users pre-load thier iTunes account in retail stores via a Passbook card. Those credits can then be redeemed for iTunes and App Store purchases, much like gift card balances work today.
Apple brings iTunes Pass to the US, other countries
The system is straightforward, although a little cumbersome because reloading your iTunes account through Passbook requires a trip to an Apple Store. Loading money to your account is handled by Apple's retail staff who scan your Passbook barcode.
It looks like iTunes Pass is rolling out in the UK, Canada, Australia, and Germany, although Apple hasn't made an official announcement yet.
[Updated to clarify iTunes Pass is a money reload system for iTunes]