Too bad the app had to shut down, but it was very predictable given Southwest's heavy hand when it comes to third party use of its flight schedules and loyalty program.
American Airlines is taking a very flexible and sensible approach to Apple's Passbook feature in iOS 6. If it takes off, then American may use the feature to push marketing and service messages to passengers.
It looks like Apple Passbook in iOS 6 will get some big traction from airlines for mobile boarding passes. If Apple goes ahead eventually with a full-fledged iTravel offering, then that could be a different story entirely.