Tibet's capital, Lhasa, is under a curfew after Tibetans fought Chinese security forces today. The New York Times has the report. I've been reading about monks making protest marches throughout Tibet, but this is the first report of violence that I've seen.
The timing is notable. China is trying very hard to pull off a controversy-free Olympics, and preemptively cracking down on a range of potential problems -- such as foreign performers showing support for Tibetans -- seems to have been one of the PSB strategies. This, and the fact that Tibetans know China will be closely watched in the runup to the Olympics by the outside world, are two key factors contributing to the unrest, in my opinion.
The wildcard is the impact of the Internet on events in China. Internet-driven dissent is a problem for Chinese authorities in urban regions, but most Tibetans don't have Internet access, and most Chinese do not sympathize with the Tibetan independence movement. According to the New York Times, Chinese-language news outlets are not reporting the riots in Lhasa. But when the news starts to leak out into unregulated, free-wheeling discussion forums and blogs in China, watch out. I believe many Chinese will demand that authorities crack down even harder in Tibet, which will put the PSB and the government in a tight spot. China wants to put on a friendly face for the Olympics in Beijing, and a bloodbath in Tibet -- spread by pictures and video uploaded to the Internet -- would result in international condemnation and protests at the Olympics.
Fifteen years ago, China could easily hide news of protests and riots. In the age of widespread Internet connectivity and cheap digital cameras, it's just not possible.
(The picture below is from the Tibetan Center for Human Rights and Democracy, India, and shows armed troops massing at Drepung, Tibet, last year after the Dalai Lama received the Congressional Gold Medal. Drepung is the location of one of the monasteries where monks have been protesting.)