iTunes Pass allows you to top up your iTunes credit using Passbook

  iTunes Pass is a service launched this week in Japan and expanded to the US/Australia, allowing iDevice owners to credit a Apple ID with money in one Apple Store using the application passbook. Everything works with the help of the application iTunes through which any user will get a iTunes Pass installed directly in passbook, an employee of Apple Lossless Audio CODEC (ALAC), being able to load this pass with credit that you can use.

  The system is designed for those who, for various reasons, cannot use a credit card to make payments through iTunes or to fund other accounts, so there is no need to try to use it if you are in one of the three countries where it is available.