A quick bit of history for those who don't know where Xinjiang is, and why this violence is occurring. Xinjiang, also known as East Turkistan, was one of the last pieces of territory to be incorporated into mainland China. It's "native" people are the Turkic Uighurs, who also happen to be Muslim. Given a choice, many of them would dearly like to break away and become a new Central Asian Republic, but of course the Chinese have no intention of allowing that to happen. Just like in Tibet, China has been encouraging Han Chinese to migrate to the province to try to strengthen their hold on the territory. They also generally allow the practice of Islam, but only within the strictures of the State structure, much as they tolerate various other groups such as Catholicism (minus the Vatican, mind you!)
I've not been to Xinjiang personally, but from people I know who have, it's not uncommon to see Han police pushing around Uighurs, and foreign tourists who look anything like a Turk are well advised to have their foreign passports well in hand to immediately identify themselves as not local Uighurs. There's a lot of hostility the other way too. It's the kind of place where most of the Uighurs can speak some Mandarin, but won't want to, and if that's the only means you've got for communication, you better be apologetic about it. The person telling me one such tale said one conversation he had in Mandarin started like this: old Uighur man: "You're not Han?" man: "No, I'm not Han" old Uighur: "Good. The Han are no good. You're sure you're not Han?" "Yes, I'm not Han." "Good." You get the general picture.
When Sept 11 rolled around, the Chinese immediately jumped on the "terrorist" bandwagon, and labeled all separatists as terrorists instead of their former labels. You may remember that there were a bunch of Uighurs at Gitmo who recently got released to various tropical islands ... I posted about that earlier, and the likelihood that the Chinese will eventually get their hands on them.