There are a few apps in the iTunes store for green screening. The best one I've used is Green Screen by DoInk and it is $2.99. You can also load DoInk Animation and Drawing for $4.99. With this program you can add animations to your DoInk green screen video.
Another is called, "Green Screen Studio". It is $2.99. This app only does pictures. You can use the backgrounds it gives you or use your own background that you save into your picture album. Once you choose a background, you just rub away the old background and the new one appears. It gets a little difficult around the edges, but it's fun to use.
Another is called, "Green Screen Movie FX". It is $1.99. I haven't tried this one out yet, but will real soon. This one allows video, as well as, pictures. There is a tutorial on you tube, so check it out.