As you all know, Internet Explorer 9 (IE9) officially released by Microsoft with many fresh features, but unfortunately, it does not support WebM videos. IE 9 requires additional plug-in to play these high quality videos.
To help you, Google has released WebM for IE9 plug-in to watch WebM videos in Internet Explorer 9 on Windows 7 and Vista. If you are an IE9 user and want to watch WebM YouTube videos, just download and install this plug-in from http://tools.google.com/dlpage/webmmf
Note: This is a technology preview version, so it might not be stable. Here at my end, IE9 crashes while trying to play WebM video from YouTube.