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 download two pieces of software: the iOS app and the OS X server utility. Open the server utility, 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. Also, isn't it a bit odd that this is exclusive to OS X on the computer side, yet it all PCs in the video? In anycase, TiltPad is yours for $US2. [iTunes]