Also having a webcast would be even better, though. Combine a webcast and, say, an IRC room, and you create a chance for the people online to discuss the speeches as they happen, fostering a community feel.

(In TransVision 06, we had that, and used a video projector to broadcast the IRC discussion to one of the walls in the room where the actual presentation was held. That led to some interactivity between the online participants and the people physical present, as comments originally made in IRC made their way to the physical world. We also included questions from online participants in the questions & answers sections of the talks.