The Carnegie Council has an interesting podcast of a lecture by Cass Sunstein on the semi-famous Colorado experiments. Those experiments tested group decision making dynamics, and showed how like minded groups tended to become more extreme as a body as time went on, the 'echo chamber' effect. Sunstein discusses the consequences of these experiments with respect to political discourse, terrorism, and blogs. A short description of the experiment by Sunstein is here.
The thing I found most interesting was the application to terrorism. Al Qaeda is described as an echo chamber, wherein discourse is deliberately pushed to the furthest extremes (normalizing suicide bombing, for instance). Combating terrorism then requires breaking up the echo chamber, pushing dissonance, not just differing information but a differing dialog. Tricky and interesting stuff.