This is not the first time that Chinese authorities blocks YouTube. In October 18, 2007, YouTube is also blocked temporarily, two week later YouTub became available again in China, but now YouTube blocked again.
The current block is actually a bit more restrictive than the other block, since any mention of the string "www.youtube.com" in any URL seems to be blocked.For example: http://www.google.com/search?q=www.youtube.com (if you are out of China, you should use http://www.google.cn/search?q=www.youtube.com) will not go through, and will get you a "Connection reset" error.
China has blocked access to YouTube in an apparent attempt to stop the spread of video footage related the rioting going on in several cities in Tibet. There were no protest scenes posted on China-based video Web sites such as 56.com, youku.com and tudou.com. The only protest scenes video is on CCTV's website. Chinese media and international media have shown footage of buildings burning and crowds damaging store-fronts.
China has at least 210 million Internet users, according to the government, and is expected to overtake the United States soon to have the biggest population of Web surfers.