Some sites are reporting that WebM is the most used format at YouTube, but that's not exactly what the blog post says. It says that they have finished transcoding the most viewed videos to WebM and all new uploads are automatically transcoded to it. It's all in the wording... and the reading comprehension.
Somewhere around 30% of all videos on YouTube are transcoded to WebM, which, as we all know, is a lot of videos. YouTube has been automatically transcoding all new videos with WebM for some time now and work is progressing on transcoding the whole catalog of videos there, but I imagine that's some years of work. I mean, 6 years of videos are uploaded there daily.
What Google said is they have transcoded the videos that make up 99% of all views and that's where the 30% of videos comes from. They did not say that most users are using WebM, just that a lot of videos are available in the format.
Basically, they have their cloud prioritizing new uploads and when uploads slow they work on the older stuff in the background until all the processing power is again needed to keep up with transcoding the new stuff.
James Zern, a Software Engineer posted this all at the YouTube blog and said they're still working to build out a full-featured HTML5 player and that H.264 support will continue so that videos still get to as many people as possible.
Patience is key in things like this as HTML5 is an emerging technology that hasn't even been fully standardized yet. Many people fail to remember that so I'm just dropping a reminder. I imagine that YouTube will be working on their HTML5 player for some time to come and the WebM format will continue growing as well.