One idea I'll steal for my next conference is the backchannel -- a chat room where everyone in the audience could participate (if they had a laptop, and most did), and the chat was shown at the big screen in the meeting room. People used it to ask questions, comment on what the speakers said, plea for a Nokia charger or whatever. A great way to interact as an otherwise passive listener. And it made even the more uninteresting parts of the conference fun.
I'd love it, but I can see others get confused by it, or totally distracted--and pity the poor. Then again, what a cool way to get totally immersed in the event. When we tried something similar for a recent small meeting, the technology backfired on us and it was a total flop, but I can see it being really fun--especially if you allow those who aren't at the event to log into it.