YouTube recently added the ability to view videos in high res, and I recently learned how to link to and embed to the high res videos directly (thanks mikeb!).
For posting/linking to a YouTube URL, add to the end of the URL:
&fmt=18
For embedding to a high res YouTube video add the following code onto the end of both URLs in the embed code: &ap=%2526fmt%3D18
The hack links directly to the MP4 version instead of