Forget about using the touch screen or accelerometers that are built into your iPhone. Instead why not use a Wiimote!
This is an impressive example of what some creative programming can be done with the iPhone.
“The video first shows the iPhone’s AirPlane mode activated before starting an OpenGL-ES Demo. In the demo, the orientation of the virtual WiiMote is controlled by a physical WiiMote over a Bluetooth connection.”
“ Talk bi-directionally between an iPhone and iPod touch without the need for jailbreaking and only using 2.0 SDK published libraries. The demo illustrates integration with Wolfenstein3D and a virtual joystick that allows for control of movement.”