Anne, I think Chris (Buenchico) answered that question not long ago.
His answer went something along the lines of it being in the best interests of the moderator to remain anonymous, therefore avoiding any possibility of 'friends' of suspended/banned members verbally attacking the moderator who they suspect might have been responsible for the suspension/ban. I do think it is up to the individual though, and not a secret as such.
Regardless of how often peeps insist there is no 'clique' on AB - friendships will be formed by folk who have common interests, etc. That's just human nature. Therefore I agree with Chris on the anonymity front.