<video> is great, but when it was first released, one of the big complaints was that it couldn’t do true FullScreen like Flash. Thankfully, this is changing and native FullScreen support is coming to most browsers in the next few months.
The API is still heavily in flux especially since the W3C joined in this week. I spent some time working through the differences to implement FullScreen in MediaElement.js HTML5 video player, and it’s working great in Safari 5.1+,
Chrome Canary Chrome 15+, or Firefox Nightly (go to about:config and set full-screen-api.enabled= true) and scheduled for Firefox 10.