Foursquare  may be many things--from a promotional advert system to a highly addictive and competitive game--but did you think it could be a tool for political dissent? The Chinese authorities think so, and appear to have banned it.
The U.K.'s Telegraph  paper is reporting that "according to some sources" access to Foursquare has been shut down in mainland China. The reasons behind the censorship haven't been explained, but it's pretty typical behavior from a government that has regularly closed access to online services when they permit users to behave in unapproved manners--Twitter  and Facebook have been targets, and the recent spat over Google 's censorship is now the stuff of media legend.
But the leading theory behind the closure of Foursquare is very simple, and it harks back to an old and massive thorn in the side of China's government (and a huge stain on the country's human rights record): The Tiananmen Square disaster. According to Chinese tech news site Techblog86  there's recently been a movement in which many smartphone users in China have been "checking in" to the 4sq.com location in Tiananmen Square in Beijing. These folks have been doing so by the hundreds as a mark of solidarity and sympathy with the dissidents who protested against government oppression all those years ago.
Inside the PRC the Tiannamen Square protests  are referred to as the "June the Fourth Incident," and authorities are sensitive to any display of political or personal disgust around the event. Indeed, Google would've been required to censor access to information on the massacre through Google.cn. And the elevated sensitivity of the government to Tiananmen Square is ably demonstrated by the arrest  just this morning in Japan of a student leader of the original protests.