Similar to apps like Darwiin Remote, which turns any Wiimote into an OS X-compatible motion controller, TiltPad can let you use your iOS device as a motion-sensitive gamepad as well.

You will need to download two pieces of software; the iOS app and the OS X server utility. Then you will open the server utility, and then open the app and the connection is opened via Wi-Fi. From there, you can configure the controls to your liking and commence gaming. You can even turn the iPhone vertically and use it as a tilt-sensitive mouse.

This type of things are for different reasons, prototyping things like this helps you build experience in electronics.