A great article over at the VideoJS blog states:
As we roll into 2011, HTML5 video hits a major milestone. 50.5% of web users now support HTML5 video playback. This number was gathered by comparing browser versions that support HTML5 video with StatCounter’s world-wide browser version statistics.
You can read more at http://videojs.com/2011/01/html5-video-statistics/
I’m exceptionally excited about this, and you should be too. We’re a step closer to being able to provide video to end users without them having to worry about plugins or codecs. There’s a long way to go, but this is certainly an improvement from the days where Internet Explorer 6 had 80% of the marketshare.
You can find more detailed information on incorporating HTML5 video with appropriate fallbacks in the book.