Safari supports it through webkitEnterFullscreen
.
Chrome should support it since it's WebKit also, but errors out.
Chris Blizzard of Firefox said they're coming out with their own version of fullscreen which will allow any element to go to fullscreen. e.g. Canvas
Philip Jagenstedt of Opera says they'll support it in a later release.
Yes, the HTML5 video spec says not to support fullscreen, but since users want it, and every browser is going to support it, the spec will change.