The fine folks over at BillGuard have a long post all about their implementation of Passbook, and some of the pitfalls.
For the sake of those of you that decided to join in on the Passbook party and create your own passes we present here some of the pitfalls we ran into. As with every new and fangled thing, Passbook is still rough around the edges and surely you’d rather work on actual features and reduce the time spent on just getting the damn thing to work, right?
This is a nice, detailed post at a developer level, not a business level. Worth a read if you want to make passes.