I have this app on my iPhone 4, and have used it inside the car, but I'm not sure how crazy I am about strapping my iPhone up higher on my car for video... if it came loose, I'd be sad.

I'm thinking about getting a current-gen iPod touch and using that instead, along with an OBD-II interface for picking up RPMs... although I'm already using that port in my Garmin. I wonder if there's a way for both devices to share the Garmin's signal.