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:
You can find the code in the usual X2 repository, under the subdirectory javascript/devicemotion and javascript/deviceorientation, respectively. Obviously, the rest of Sencha Touch library is needed before you can deploy and run the examples. For the lazy, just run the live demo for Device Motion and Device Orientation (they support offline mode).
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?).