As of yet, these reports are unconfirmed, but the word on Weibo is that workers at Foxconn (a supplier of Apple and just about every other tech company) rioted at a factory in Taiyuan. (Thanks to @niubi for the heads-up on this one). It is not clear yet exactly what's going on, but China's social networks are full of images like these:
There's also this video (h/t to Richard Lai for this), although it appears to have been shot after whatever rioting took place is over. It's dark and shaky, but if nothing else it's certainly evidence that an awful lot of people were out late at night. Rumor has it the riot started when Foxconn guards beat someone, inciting the wrath of employees from Shandong and Henan. Beyond that, not much is clear at all. We will keep an eye on this story, but until it's confirmed please remember that these are just reports from Sina Weibo, so they may turn out to be inaccurate.