Vokle tries to let you do exactly that. It boardcasts one user that is hosting an "event" - that is presenting something. Other users can then enter questions or comments via text, or virtually raise their hand and the presenter can then allow them to ask the question via voice/video.
While the fact that only one person (plus one possible extra from the audience) can speak at a time, it actually solves a few issues with regular video conferencing. One no longer have to deal with the chatter and murmur of everyone in the group. Instead, each will participate only when it is useful and they mean to do so.
