I know there's a few of my British friends who've been waiting on this one; British Airways has finally added Passbook support to their iPhone app! The update is pushing out through the App Store as we speak, and going forward all boarding passes will be presented in Passbook. There is however an issue with the app for anyone on iOS 5, and BA is advising users on this software version not to update at this time.
The Passbook integration isn't quite finished yet, though, as BA is announcing today as part of the update that they're going to make it even better later on in the month. Passbook boarding passes will be enhanced so that they're presented to you on your lockscreen as you're ready to fly. When this is ready to go, customers will be notified by an in-app message. So, it took a while to arrive, but when it did, it's done right. Good job British Airways!