Why do they ask this? Surely no one ever says no do they? Even if you're not 100% sure it's your dream job, you wouldn't go for the interview if you didn't want it, would you?
I've just been asked this in an interview.
Am trying to keep my mind off it for the rest of the day, obviously by picking apart the poor interviewer! I was the first of 6 to be interviewed and have another interview later for something else! A whole day of nerves for me then!