This is a major shift from what Microsoft said before about HTML5 video in Internet Explorer 9. When the company detailed its HTML5 video support, the company was very adamant in that it would go all-in on H264. It would be the only video codec supported by Internet Explorer 9, no matter which codecs the user had installed. Most likely pressured by the prospect of a VP8/WebM-only YouTube, Microsoft has now given in.
"In its HTML5 support, IE9 will support playback of H.264 video as well as VP8 video when the user has installed a VP8 codec on Windows," writes Dean Hachamovitch, General Manager, Internet Explorer. This puts support for VP8/WebM in Internet Explorer 9 on the same level as support for H264, with the only caveat being that Windows ships with H264 out of the box, whereas support for VP8/WebM has to be installed separately by the user.
This leaves only Apple's Safari out of the loop. While Google announced plugins for Gstreamer, and has already made the DirectShow filters available, there's no peep yet about QuickTime plugins (Safari's video falls back on whatever QuickTime can decode). It will likely be up to a third party or Apple itself to implement support for VP8 in QuickTime.
Times are getting interesting.