One of the most interesting question is, when Google's own product, Android, is supporting WebM. Of course it is very important to have that format at the mobile alternative to the competition. Google officially said it will be supported in Android "Gingerbread". This is scheduled for Q4/2010. And this is quite long. In addition, "old" Android-version can't use it.
But wait a second, not so fast. Why should it be like that? Integrated in Gingerbread does mean supported from the system. There is no reason why there shouldn't be an video-player that support this format to download for almost any version of Android.
I recently found am app to play XviD and some other formats directly. Some of them not supported by Android. So my guess is, there is no reason why there shouldn't be an player and browser-suppoort for alternative browsers, on Android 1.6 - 2.2. Time will tell.
To answer upcoming questions why there isn't support right now, I have to say, the WebM-Codec just came out. It isn't ready for prime time yet. This will take some month. So, Q4 this year is quite fast, if you think of it. This would be the fastest integration of a codec on a mobile device ever.
Photo Android with Gingerbread found on Geekword.net . CC BY-NC-SA