One of the things I always do when I moderate a panel at a conference is encourage, in the strongest possible language, the audience to break in with their questions anytime during the presentation. It's the very best way to make sure that you're providing the exact information that attendees want. Even so, it is rare that people speak up. That may be because I work in the financial services industry and bankers tend to be more conservative, but it still makes it more difficult to gage whether the audience is getting what they came for.
Now, New Media tools are making it easier for conference attendees to be part of the conversation without actually saying anything. While this is prevalent now only at technology- or New Media-focused conferences, many execs are now using Twitter to make comments during the session. A person provided by the conference or the session moderator monitors the tweets during the session and passes the comments and questions through to the panelists.
It may take a while for this to filter down to non-tech-related conferences, but it's already becoming prevalent for online event, such as webinars. In fact, one company, Radian6, has launched a series of twebinars (twitter mashed up with a webinar) hosted by Chris Brogan that allows attendees to discuss the event in real time before during and after the session.
I expect these tools to become mainstream for all conferences relatively quickly, perhaps as early as next year. I base that on my experience at last week's 2008 Predictive Methods Conference in San Diego where attendees were asked to interact with conference speakers by keying in answers to questions on a Funnies-Home-Videos-style keypad. The results were displayed graphically for the forum in a matter of seconds.
People love to interact and when technology makes it easy to do so without saying a word or drawing any attention, it's going to enjoy rapid adoption.