Even though Apple’s (AAPLiOS 6 and its highly-anticipated Passbook mobile wallet app haven’t been released yet, airline companies are already allowing mobile boarding passes to be saved to the developer build of Passbook for iPhone, reports Australian Business Traveller. The publication cites a reader who was traveling on Virgin Australia and was prompted to add his boarding pass to Passbook on the developer build of iOS 6 installed on his iPhone. So far, United Airlines is the only airline that’s been officially announced as a Passbook backer, but more will likely follow suit as Passbook becomes more widely available.