Here's an interesting risk management scenario: What would you do if one of your general session speakers was stabbed by an attendee after his presentation? That's what happened at the 10th International Conference on Science and Consciousness held earlier this month in Santa Fe. Read more about it here.
I don't think there's much a planner could do to keep this from happening. Fortunately, it sounds like there were plenty of medical folks in the audience, and the paramedics showed up quickly, but sheez.