"You are in a trial for HTML5 video on YouTube. Some users of supported browsers are automatically entered in to the trial. If you wish to leave the trial, use the link at the bottom. HTML5 is a new browser technology that allows us to show videos without the Flash plugin," explains YouTube.
If you right-click on the video, you'll see a menu that offers two options: "save video as" and "about HTML5". You might assume that "save video as" lets you download the video, but it actually sends you to this video.