The magic of Google Books reveals this chapter on how Twitter helps at conferences by "micro-connecting a diverse online audience."
It's easy to make fun of Twitter. Really easy. But this chapter makes a pretty lucid argument as to its usefulness and, I admit, I'd never thought of it as a pre-conference, during conference, and post-conference tool -- probably because I avoid conferences. The learning part, the what people know, is obvious. We instantly know what people think about what someone is saying during a conference rather than waiting for later in the bar over drinks to hash it out. That's the good side, though tweeting constantly probably also means you miss a lot of what is said.