Movie FX Cam is the first application I have found that does that. The app itself is a bit crappy. Let's get all the negative out of the way so we can concentrate on the good stuff. It crashed the first time I tried to use it but haven't done it since so it is usable. It has very annoying welcome screen and you have to "Touch to start" to get to shooting. I don't see the point and it just makes it slow to start. It has low output resolution of 2048 x 1530 px compared to native cameras 2592 x 1936 px and it could use a lot of usability tweaks.
The app can be used to shoot movies as well but I'm not interested in that so I only shot stills. Movie FX Cam has many effects but I have been mostly using 'Film Noir' effect which I like a lot. I shot a funeral once with Leica M6 + pre-asph 35mm Summilux and Kodak Tri-X which was pushed couple of stops in development. 'Film Noir' effect looks a lot like those pictures. The fake film grain is the best I have seen in any iPhone application. Pictures taken in the dark show blotchier grain but that's because the iPhone camera sensor can't handle low light and produces actual noise.
More of my iPhoneography can be found here.
Here's few samples taken with Movie FX Cam and iPhone 4: