Google announced this week in a blog post that they have started to convert videos on YouTube to the WebM format more aggressively, moving away from Apple backed H.264. According to Google until now approximately 30% of their catalog is already available in the WebM format, which make up 99% of the views on the popular website.
In order to convert the remaining videos at a rapid pace, Google leverages their cloud infrastructure, whenever computational capacities are free. At busy times the WebM transcoding is paused to free resources for users uploading new content. For now Google intends to continue supporting H.264. This is not surprising as it is the only codec usable on mobile devices from Apple like the iPhone and iPad (and since there is no Flash allowed on these devices basically the only means to watch YouTube).
Also a lot of devices have been equipped with hardware acceleration for H.264 which allows for power savings and in some cases enables the playback of HD content that would otherwise not be possible. For WebM hardware acceleration support is non-existant at this point. If you are interested in testing HTML5 video on YouTube, it is possible to opt in at the HTML5 page