My friend Poodles has a post up on her blog here on a topic of interest to atheists.
While it may not happen often, it is inevitable for just about all of us that we are going to end up having to converse with a religious figure such as a priest or a rabbi.
As someone who was raised and was once a believing Catholic, the most uncomfortable for me is to have to speak with a Catholic priest, because they are generally addressed as "Father." As an atheist, I cringe at the thought of addressing a man as "Father", because to do so in my mind implies that I am acknowledging that he has some sort of spiritual overlordship over me. I generally tend to refer to priests as "Sir", which is what I did during my father's funeral a couple of years ago.
On the other hand, I don't think I would have a problem addressing a rabbi as "Rabbi", though I would probably say it in a rather matter-of-fact tone devoid of any reverence.