From You Tube Blog
We’re excited to say that support for watching 1080p HD videos in full resolution is on its way. Starting next week, YouTube’s HD mode will add support for viewing videos in 720p or 1080p, depending on the resolution of the original source, up from our maximum output of 720p today.
As resolution of consumer cameras increases, we want to make sure YouTube is the best home on the web to showcase your content. For viewers with big monitors and a fast computer, try switching to 1080p to get the most out of the fullscreen experience.