Police in Essex, England have charged a 20-year-old man with "encouraging or assisting in the commission of an offence" (under the 2007 Serious Crime Act) because he used Blackberry Messenger to encourage people to attend a public water fight. It's not clear whether the police were working from an informant or whether they have developed the capability to wiretap Blackberry's notionally encrypted messaging network (I'm not clear whether Blackberry has the capacity to decrypt and read messages, or whether the encryption is end-to-end.)
In 2008 there was a spate of mass water fights in British towns and cities that were organised through social networks. Most remained peaceful.This month a water fight attended by thousands of young Iranians attracted the attention of Tehran's morality police and led to a series of arrests.
Prime Minister David Cameron has proposed a national censorship regime to block or filter the Internet to prevent social unrest (this despite the failure of the Chinese government to effectively manage the trick with vastly more resources and expertise and vastly fewer legal constraints). One week before this proposal, Cameron's government rejected the Digital Economy Act's provisions for censoring the Internet to prevent copyright infringement, having concluded that such censorship regimes were easy to evade and would not be effective.