DELTA, when iTunes updates the apps, the old versions get moved to the trash bin. You probably can find your old AirVideo version there.
For the time being, I reverted back to the old one as well. And I'm keeping my phone on 4.2.1 until these issues can be ironed out. I knew Apple would leave 3GS users out in the cold sooner or later, but I didn't think that would happen before the iPhone 5.