If you're not already in Tibet today, it's going to be a little while before you're allowed to enter the region. The Chinese government has stopped issuing the special permits that foreigners require to enter Tibet and won't let any tourists in again until October 8.
This kind of behavior is, of course, nothing new. China spent decades banning tourists from Tibet until it was opened up a bit in the 1980s, and banned foreigners again in March 2008 in response to rioting.]]>