Over 50% of web users now support HTML5 Video
videojs.com
videojs.com
You can see a comparison of mobile OSs here: http://gs.statcounter.com/#mobile_os-ww-monthly-200912-20101...
It'll be a bit until WebM chipsets come out for the incoming flood of Android tablets, and since h.264 is the easiest target, I can't see the current statistics really influencing the encoding of video until Android can make a dent. Flash video works great for the 90% of computers running Windows and the plugin, so why fix something that works?
I hope that idea you had is no longer still itching to get out. There.
Though that starts to look better if you consider good adoption of IE9 and Firefox on Windows with Microsoft's plugin.
Seems like enough to draw conclusions from.