Youtube has been working on getting to you HD versions of your updated videos for some time now ; but they havent rolled it out in public. Apparently, the work is so advanced that most of the videos that you upload are indeed processed into their HD variants and a simple hack allows you to view the same. Note that its not sure if this works with older uploaded videos (if you uploaded them recently, it works for sure) and also that the resolution of the uploaded video should not be any lower than 720p.

As for the hack itself, all you need to do is add "&fmt=22" to the end of the video URL. Youtube does take longer to process the HD video and it may not be available immediately upon upload. Try it after 20-30 minutes and you should get it to work.

Viewing HD videos will require you to have a high end machine since these videos are pretty CPU intensive, so just make sure you have a powerful processor and plenty of RAM.

HD View:

Normal View:

Obviously getting the embed codes for the HD version isnt very easy since there is no direct way to do it, but Josh from Cnet seems to have figured it all out. Any further queries to be directed to him.

Source