I did the iOS 6 upgrade today to my creaky old 3Gs and everything works fine. At IAD now and used Passbook for the first time in connection with United's mobile app. Worked great. Pull up your boarding pass within United's mobile app and there's a new gold button (bar actually) that says "add to passbook." Boarding pass looks great in passbook. All flight information and Q code are visible without having to scroll.