Tiltboard, a mod for the Xbox 360 controller that swaps the use of the right analog stick for relative motion-based controls similar to the Sixaxis and Wii remote. Switching between the motion controls and the analog stick, as well as calibration and sensitivity adjustment, are done via two buttons that stick out of the back of the controller. Creator Adam Thole sent Joystiq a picture of the back of a "Tiltboarded" gamepad, shown above, this morning.
The mod cost $39.99 and unfortunately right now it comes as a kit for you to install yourself. But for those really longing for motion-based simulation in their Xbox 360 titles, this is probably your best bet for now. The Tiltboard will start shipping next week.