For almost as long as there's been conferences there's been conference back-channels. The precise medium which forms the back-channel has morphed over time, from quickly scrawled notes passed amongst delegates, to SMS messages, to IRC (Internet Relay Chat for those of you old enough to remember what this is). With IRC, the back-channel became a conversation, recognisable amongst conference goers. Witty, informative, scathing, irreverent, the back-channel provides near real time information on how the conference is going and on how the current speaker's presentation is being received.
Which brings me to Twitter. These days Twitter has all but supplanted almost every other form of back-channel communication. Not every conference venue and conference organiser likes this. I was recently at a conference which provided no network connectivity in the conference hall at all. When questioned, the excuse was that "using laptops distract from what the speaker is saying". Ignoring the fact that 3G data dongles and smart phones are pretty much ubiquitous these days, it does make live demos and live blogging just a tad challenging. At the opposite end of the spectrum, some conferences actively encourage the Twitter back-channel, going so far as to publicise the official hashtag to be used and providing large screens running Twitterfall to provide immediate feedback to speaker and audience alike.