http://blogs.msdn.com/b/ie/archive/2011/03/16/html5-video-update-webm-for-ie...

It appears to be a little rough around the edges, but should bode well for Wikimedia video support as IE 9 starts to be pushed out to windows machines and ideally we won't have ie7 and 8 for as long as we have had IE6 ;)

If you have not already seen it, the TimedMediaHandler extension supports transcoding to both webm and ogg for mediawiki video assets: http://prototype.wikimedia.org/timedmedia/Main_Page

I will integrate links to http://tools.google.com/dlpage/webmmf/ for IE9 users in the mwEmbed player once the components are working a bit more smoothly.

peace, --michael