It could be possible to make a client application for a game (racing game or similar would be the best example) that provide the accelerometer input to your mac to control your car. We have just seen in the WWDC2008 keynote some games on the iPhone that use that input. Providing it to a game could be funny… You move your iPhone to steer, accelerate or decelerate holding your left or right thumb on the screen and change gear with a sudden pitch (rotation forward and back – or back and forward on the x axis). You could also have an area on the iPhone to pause the game…
I hope the image is clearer than what I have written down…