As Tibet transitioned  into total lockdown and videos of the violent situation  proliferated on YouTube , people began noticing Saturday afternoon in China that the video-sharing website could not  be accessed.
Tech blogger Rick Martin on the CNET Asia Little Red Blog has done some tests  which confirm what many have assumed:
There were some videos uploaded to Youtube already about the demonstrations, but this block will definitely throw a wrench anyone's plans to upload more. That said, Chinese versions of Youtube, which have been told to censor this kind of sensitive content, are all still up and running as I write this.
At the same time, other tech bloggers have noticed increased sensitivity  online in general recently.
Which begs the question  of authorities in Beijing: just how stupid do you think people are?
March 17 update:
Jeremy Goldkorn at Danwei.org has been following closely  reports of further problems people in China have been having online:
Youtube is blocked, the websites of The Guardian and L.A. Times newspapers are not currently accessible in Beijing; we have reader reports of the Yahoo home page not loading from around the country.
March 23: Bloggers in China began reporting  Saturday that YouTube can once again  be accessed. However, as Martin points out  and others have noticed, what's being seen at present is only a partial recovery.