I already showed the use of device orientation in a fun game called Box of Marbles. Based on my experience trying out this API implementation, often it is easy to get confused by device motion vs device orientation. Thus, I created two web-based applications (using on Sencha Touch): Device Motion and Device Orientation. Each basically just visualizes the values of the data in a bunch of sliders:
While right now only iOS 4.2 on iPhone/iPad/iPod Touch implements this API, I have a strong hope that Android (post Gingerbread) and Nokia (thanks to QtWebKit and Qt Mobility) will support this cool API in the near future. Well, at least I do plan to revive my Symbian setup to custom compile QtWebKit and test it on Nokia N8.
Side note: from my limited understanding, seems that both accelerometer and gyroscope are needed in order to supply the accurate data, much like in an inertial navigation system ("Inertial Phone", anyone?).