A post over at the Windows Blog by Dean Hachamovitch, general manager of Internet Explorer at Microsoft, brings word that IE9 will in fact support the new WebM video system announced today by Google with backing from a broad range of hardware and software companies like Mozilla, ARM, Skype, Oracle, Opera, Adobe, AMD, Broadcom and others:

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.
-Dean Hachamovitch, General Manager, Internet Explorer

Emphasis mine, and it makes all the difference in the world. They aren’t pledging to ship a WebM codec package with IE9, but at least they’ll support the codec if users have it installed. Even with that caveat, this is a reversal of Microsoft’s previous position that h.264 would be the only codec IE9 would natively support inside the video tag, citing the need to support a common platform developers could rely on:

Developers have consistently conveyed that they want certainty and predictability in the underlying browser platform. We want to deliver a great HTML5 experience in IE9 with great certainty.

Allowing the video tag to rely on codecs that may or may not be installed is not consistent at all, which is why they essentially said a few weeks ago “if you want to use other codecs, build a plugin”:

To be clear, users can install other codecs for use in Windows Media Player and Windows Media Center. For web browsers, developers can continue to offer plug-ins (using NPAPI or ActiveX; they are effectively equivalent in this scenario) so that webpages can play video using these codecs on Windows. For example, webpages will still be able to play VC-1 (Microsoft WMV) files in IE9. A key motivator for improving the codec support in Windows 7 was to reduce the need that end-users might have to download additional codecs. The security risks regarding downloadable codecs and associated malware are documented and significant. By building on H.264 for HTML5 video functionality, we provide a higher level of certainty regarding the security of this aspect of browsing and our web platform.

http://blogs.msdn.com/ie/archive/2010/05/03/follow-up-on-html5-video-in-ie9.aspx

They were throwing their weight behind h.264 entirely, but now that has changed, they can’t just ignore WebM at this point, not with the huge list of industry backers the project has at launch, but they aren’t exactly committing to ship a WebM codec package with IE9 either, at least not yet.