Since iOS 4.2, Mobile Safari supports accelerometer events. The native browser on Android doesnt do that yet. (I've heard that it does accelerometer in WebViews, ex via phonegap, but i couldn't confirm this now) Firefox 4 (currently in Beta 13, 28 March 2011) does support accelerometer events! Of course, this is awesome, not least for building games on websites!

So, this is how you do it:

You can view the demo here: http://nerdcommunications.com/demos/accelerometer/

You can download the demo source here: http://nerdcommunications.com/demos/accelerometer/demo.zip