Eric P. Robinson, writing for the Digital Media Law Project:
A British judge’s decision that a tweet by Sally Bercow (wife of the Speaker of the House of Commons, John Bercow) libeled Lord Robert Alistair McAlpine (former Deputy Chairman and Party Treasurer of the Conservative Party and an aide to Prime Minister Margaret Thatcher) shows – if anyone still had doubts – that tweets can indeed be libelous. In doing so, the ruling provides a good model for analyzing Twitter posts to determine whether they are defamatory.
This is an interesting case - one that shows great responsibility can fit into 140 characters.