I recently posted on Almost DSLR, a free video camera app for ios to give you more controls over your video. Of course this does give you decent controls over your exposure and focus, with the ability to lock them.
If you want more control over your filming with an iPhone (and you don’t mind paying $2.99 for it) Filmic Pro looks to be a great app.
I have not purchased it, but it is very tempting. It’s best feature in my opinion is being able to select the frame rate you want. It also includes manual controls and audio meters.
Gizmodo posts a pretty in depth review, so if your interested in the app check it out.
I don’t often use my iPhone as a video camera during production in any of my videos (maybe a clip here or there or for behind the scenes footage). So for now I am going to stil with the free app, but down the road if I use my iPhone for video a bit more who knows.