I'm sitting in the hotel lobby with Brian and Stephen, putting some thought into wtf we're going to try during our keynote. One idea Stephen had was to have an open web-based chat room, so attendees (physical or virtual) could ask questions, make comments, etc... without the intimidation of grabbing the microphone.
After a quick Google, and a few false starts, I found a link to Lace - it's a simple PHP + "Ajax" chat server, using flat files to store session and chat data. That means it can run anywhere PHP can run, without needing a database.
I put a copy on one of our servers for us to use during the keynote. It looks like it will be much more open than an AIM group chat, and easier than an IRC channel.
We'll also be mobile - the three of us with wireless microphones - and there will be 2 standing microphones in the room, so audience participation should be relatively easy. If the folks aren't into it, we'll fall back into a more conventional panel discussion, but we're really hoping that the attendees take over the session.