Pearson (the educational publisher) has just brought out a document entitled 'Tweeting for Teachers'. The document is available to download from http://bit.ly/nXEqMY.
It references some individual developments that are impressive. The #ukedchat twitter conversation that takes place every Thursday evening is perhaps the most convincing demonstration of teachers using social media in an organic and widespread manner.
The report does have weaknesses; most notably that the authors have not drawn from two very rich sources of evidence. These are the fields of Knowledge Management, and that of Communities of Practice. The work of Etienne Wenger is very significant in relation to the latter, and Max Boisot in relation to Knowledge Management (see for example, 1995, Information Space: A Framework for Learning in Organizations, Institutions and Culture, London, Routledge). Wenger maintains a website http://www.ewenger.com with ample references to his work if you have an interest in pursuing further reading.