Nice video about a history professor in the States using Twitter as a way of holding class discussions with 90 students. They found out a lot of the same things that we've discovered with I'm a Councillor, I'm a Scientist and ConferenceXtra – being able to contribute digitally suits some people better than having to talk in front of 100 people. And the trail of comments allows people to follow up at a later date.
Jeremy Hardy likened Q&A sessions at literary festivals to Guardian reader karaoke. Some people do like to go on under the pretence of asking a question. 140 characters puts the focus on conciseness.