Is this really a hack as some reports are labeling it? Not really. The extension &fmt=18 is purely visible on the link to watch the higher quality. However if the link “watch in high quality” does not exist, you can get to it by adding the extension.
I found a great example of the higher definition that YouTube is offering. Open the two Paul Pierce videos below from the NBA finals in two separate windows. The first is the “standard quality” which has much to be desired. The second video is the higher resolution. Compare the score graphic at the bottom and the close ups to really see the difference. Not “high-definition” by any means, but still worth reporting on.
Standard quality
http://www.youtube.com/watch?v=ml4-TXfMwnM
Higher resolution quality
(notice the addition of “&fmt=18”)
http://www.youtube.com/watch?v=ml4-TXfMwnM&fmt=18