I just found the best movie app on the iPhone and it's called Super 8. iPhone users love to take pictures. And they love to shoot video. Super 8 will appeal to those types of people. Super 8 also takes advantage of the retro style that is "in" at the moment.
Now, I'm a little too young for the generation that actually used Super 8 film cameras. Still, this app has the right feel. It feels like an app that one could charge one or two dollars for. Instead, it's free.