From CyberNet article:

"The words YouTube and “high-resolution” have never really fit well in the same sentence. YouTube has always greatly limited the resolution of their videos to conserve on bandwidth, but going forward that may not always be the case. As it turns out there is a little something that you can add on to the end of a YouTube URL to have it play a high-resolution version of the same video."

The story continues sharing a way to test:
Ready for the example? Just add &fmt=6 onto the end of any YouTube URL. Using the skateboarding dog as an example you would take the normal URL:

http://youtube.com/watch?v=CQzUsTFqtW0

and add the &fmt=6 onto the end:

http://youtube.com/watch?v=CQzUsTFqtW0&fmt=6

http://cybernetnews.com/2008/02/29/watch-high-resolution-youtube-videos/

I see no difference

Juha-Matti Laurio:
It doesn't work this way for the embedded code. Add that parameter to the YouTube page.

For an embedded video that works, check:
http://googlesystem.blogspot.com/2008/03/youtube-tests-higher-resolution-videos.html

Yeah, you have to add '&fmt=6' manually or click the URL generated by this forum application after the embedded video sections.

oh right, sorry, it's cool