Google has released a preview of the WebM Video plug-in for Microsoft Internet Explorer 9
WebM for IE9 is a free browser plug-in that enables support for playing WebM videos in Microsoft Internet Explorer 9 on Windows 7 and Windows Vista.WebM Video for Microsoft Internet Explorer 9 (Preview)
Note that this release is a technology preview. See known issues.
They said elephants couldn't ride flying dolphins. They said that one of the world's most popular browsers couldn't play WebM video in HTML5. They were wrong.