Computer scientists in the US have discovered ways to “re-identify” the names of people included in supposedly anonymous datasets.In one example, a movie rental company released an anonymous list of film-ratings taken from its 500,000 subscribers. Using a statistical “de-anonymisation” technique, the academics were able to identify individuals and their film preferences.
http://www.guardian.co.uk/technology/2010/jan/24/computer-security-crime-anonymous-datasets