I’m not sure who got to it first but Dailymotion, Youtube, and my site (http://NumberedHumanIndustries.com) are the only sites that are using the HTML5 video tag.
Dailymotion’s page is at http://openvideo.dailymotion.com and as far as I can tell is using the Vorbis/Theora Firefox 3.5 compatible spec.
Youtube’s HTML5 site is at http://www.youtube.com/html5 and is using H.264 and doesn’t seem to work in Firefox 3.5
My HTML5 page is http://NumberedHumanIndustries.com/video and it is Vorbis/Theora and works in Firefox 3.5
If you know of any other pages that are using are using the proposed HTML5 video tags please comment here, email me three [at] threethirty [dot] us, or @threethirty on identi.ca
Free Your Software/Culture and the Rest Will Follow 
http://www.dailymotion.com/html5 does not work on FF 16, it works on Chrome though.