I work for a consulting firm and was recently on a conference with a client and one of my colleagues. The client asked me a question and I said that I wasn't sure and that I'd find out for him. After we rang off the colleague said that I should never admit that I don't know something. I'm supposed to be the expert.
It has, however, been my experience (I'm 25 years older than this guy) that the customer will respect you far more in the end if you admit you don't know something than if you BS your way through an answer.