From MIT Media Lab, (Thanks, Howard!):

TXTmob is a free service that lets you quickly and easily broadcast txt messages to friends, comrades, and total strangers. The format is similar to an email b-board system. You can sign up to send and receive up-to-the-minute messages from groups of people organized around a range of different topics. TXTmob was first used by activists protesting the 2004 Democratic and Republican National Conventions. It was also deployed during the Ukranian Orange Revolution and by demonstrators at the 2005 inaugration of George W. Bush.

And Beijing, Shanghai, Shenzhen, etc…..