http://news.bbc.co.uk/2/hi/asia-pacific/7961069.stm

"China is reported to have blocked the YouTube video-sharing website because it has been carrying video of soldiers beating monks and other Tibetans.

The date and location of the footage, posted by a Tibetan exile group, cannot be ascertained.

A Chinese government spokesman would not confirm whether YouTube had indeed been blocked.

China has a history of blocking websites which carry messages it views as politically unacceptable.

In most of China, YouTube has suddenly become inaccessible.
...."

Reuters states that 'Access to YouTube had been spotty earlier in March, the one-year anniversary of widespread protests by Tibetans against Chinese rule'.
http://www.reuters.com/article/internetNews/idUSTRE52N1VN20090324