Google/Toutube last night released an opt-in option of the Youtube.com site that plays videos by default in HTML5 instead of Flash. The opt-in only works at Youtube.com, not on embedded Youtube videos. Also, Chrome and Safari are the only browsers supported at this time (no Firefox or IE).
If you want to stay Flash free on embedded Youtubes, you can use ClicktoFlash.
Adobe can’t be pleased with this. via DF
Update: Unsurprisingly, commenters report seeing significant CPU usage drops when using HTML5 vs. Flash.