Yesterday I uploaded a video to YouTube. The file was rendered to HD (for vimeo, actually), but as it turns out, YouTube now offers to watch it in 'standard quality' and 'high quality'. Check this out: 
This is a pleasant surprise, of course, so I want to know more about this. Unfortunately, I cannot find anything about this in the YouTube help section. Can anybody direct me to information?

This is a pleasant surprise, of course, so I want to know more about this. Unfortunately, I cannot find anything about this in the YouTube help section. Can anybody direct me to information?