When Apple introduced Passbook in iOS 6 they billed it as a way to store all your tickets, coupons, boarding passes, loyalty programs, and other cards all in one place. It's hard to imagine they expected that place to one day be Microsoft Wallet, but thanks to Windows Phone 8.1 that appears to be exactly what's happening.
Apple code-signs their Passbook cards, and it's not yet clear if Microsoft is simply accepting Apple certificates or accepting any pass regardless of whether it's signed or not.
Apple also provides a push-notification-based service to update cards, like gate numbers on boarding passes or balance info on Starbucks cards. Windows Phone can read and render the .pkpass bundle, but they probably can't hook into Apple's push system so, if updates are possible, there'd have to be some Microsoft-specific support from the supplier, or laborious polling process in place.
In a perfect world a system like Passbook would be universal and everyone could use and benefit from it no matter who manufactured their device or built their operating system. That's how we get standards and standards is how we get mass market adoption.
From Apple's point of view, however, Passbook is a feature that adds to the value of the iPhone and the experience of iPhone customers.
If there's no official agreement going on here, it's possible Apple would put a stop to Passbook support when they find out about it and if they can, technically or legally. If there's some form of cooperation going on, or some future agreement to be had, that could end up being a good solution for everyone, from the companies to Apple and Microsoft to us, the people who want these kinds of services made ubiquitous.
What about you — do you like the idea of Windows Phone 8.1 working with Passbook?
Daniel Rubino contributed to this story.