Flying vehicles in this sort of game are a nightmare to code. You've gotts start creating whole new draw distance routines and design the world in a way that would allow it to be viewed from any angle plus have collision present everywhere. Borderlands already has a fondness for invisible walls and limiting you're exploration to help save memory usage so chances are a flying vehicle would cause all kinds of memory issues.
http://i24.photobucket.com/albums/c6/lvl54spacemonkey/Tink.gif Le 360 generale. http://www.gamefaqs.com/boards/2000325-xbox-360-general-social