The new HTML5 Video-Standard from Google is now supported by Android.
In Gingerbread (Android 2.3) there is full support of playing WebM inside HTML5-Pages. Just like I expected, it took only a short time. However, it's not any solution in sight for devices prior to Android 2.3. It should be possible to develop an external player.
This is a major step. Mobile video is considered a big thing in the 2011. WebM is ready.