New orientation device API for Firefox 3.6 look promising
Especially if you would like to develop a web OS tablet a la Palm PRE WebOS, or game based on environment. See here a Mario Kart game based on this new orientation API. Unfortunately only some modern Mac book and Lenovo ThinkPad have the sensor..
Original post at http://hacks.mozilla.org/2009/10/orientation-for-firefox/
Documentation Device orientation documentation