Which means, somewhat surprisingly, that IE could be 3rd globally in the relatively near future. In fact the IE drop/Chrome rise appears to be accelerating, so it could happen sooner.
http://gs.statcounter.com/#browser-ww-monthly-200812-201012
But that's all versions of IE, IE9 won't be available on XP, and historically it's taken years for new versions to penetrate, so it'll be a much weaker force than Chrome and Firefox which complete updates in a few weeks and months respectively.
Microsoft getting on board would be nice, but Adobe getting WebM into Flash would take care of the older versions of IE that are hanging around too.
I certainly wouldn't discount it. MS made the H.264 decision well before Google announced WebM. It wouldn't shock me to see them cut a deal with Google if for no other reason than to put Apple at a competitive disadvantage.
Except Internet Explorer will continue to be the highest-marketshare browser for the foreseeable future, and IE9 won't ship with VP8 (though you can install a codec for it, I doubt regular people will). It does ship with H.264.
Maybe Microsoft can be convinced to ship VP8/WebM in IE.