YouTube has a major challenge with their adoption of HTML5, they want to leverage the new format and capabilities, but they don't want to make it easy to download videos. This is going to be one of the biggest challenges youtube faces until they realize that allowing users to download videos will not reduce their market position or decrease ad revenue.
Originally with the flash version, downloading youtube videos was as easy as copying the unstreamed files from /tmp/ assuming you are using linux. With the introduction of HTML5 videos (both .h264 and webm videos) they made it a little bit harder, because HTML5 videos were not stored in adobe's cache, but in your browsers further obfuscated cache. This was overcome by disabling a content blocking box that they put over the video.
Round 2 of Downloading YouTube Videos