If you do a lot of moviemaking on your iPhone, you'll probably want something with extended capabilities. There are plenty of apps to help at various price points and feature sets.
Recently-released MoviePro is among the better ones. Although it runs on the iPhone 4 and the iPod touch, you don't get all the features without the more powerful iPhone 4S.
MoviePro offers pause and zoom (4S only), selectable frame rates, aspect ratio, and variable audio and video quality. There is a tap-to-focus feature (additional taps set exposure), auto white balance, a timer and the ability to choose maximum record duration. The zoom feature just magnifies the pixels you already have, but some people will find a use for it.