But your browser is already downloading the video that's how YouTube works. You just have to find the direct link to the file, not that hard.
This video (http://www.youtube.com/watch?v=r3FJwuykNoc&feature=popular) has a link underneath to download it for $0.99. I looked at a few other videos and i think they are only offering to download HD videos. Any body know when/why this started, or if google plans to start an itunes like store? I dont know why anybody would pay to download youtube videos, but this is interesting and any info would be appreciated. Thanks.