How about using something like maesum#gps. Each presentation would have this in a corner of the displays somewhere. Might be tricky if people want to do a demo directly from their device, unless we use overlay.

Another idea would be to have two projectors per talk. One projector would be the host's screen (slides, demo, etc), the other screen would display live feeds from twitter/internal Summit protocol that could display questions as they get fed into the system. The question fades after a few seconds, and at the end of the talk, when there's a QA session, the host can go through the questions sequentially. If there is no QA session, the questions could be automatically stored to the wiki relating to that talk.