From LedHed's Wiki
To link a youtube video so that it opens in Hi-Def (1080) append "&fmt=37" to the end of the link.
&fmt=37
For example:
Normal Link
http://www.youtube.com/watch?v=ysSxxIqKNN0
Hi-Def Link
http://www.youtube.com/watch?v=ysSxxIqKNN0&fmt=37
To link to at 720 resolution use:
&fmt=22