Being put on the spot
Ted Neward: You never bullshit. You admit that you don't know the answer and then say, "But I think I can find out and I'll get back to you if you want to know more." I also think that it's worthwhile to take a guess and say, "Based on what I do know, I believe that the answer will be this and if you'd like to, we can do a little research and see if we can figure it out."
In that way, you actually have a new contact in your network, whereas if you knew the answer, you probably would have given the answer and would never have spoken to that person again. Truthfully, nine times out of 10, when somebody at a conference asks you a question, it's because they're speculating. They're not looking for the answer; they're just trying to see how it fits within their mental model.
If they get the answer they expect, even if it's not guaranteed to be accurate, it helps to solidify the mental model in their head...