Why do you think it would be a good idea to tell school B, with whom you are wait listed, that you have another offer in hand? It could be a good or a bad thing to do; but either way, i would stay silent.
1. You are waitlisted! They don't have to offer you anything, ya know? I see it like this. If I see you, someone who has an offer, and another person who is identical in every way who doesnt, I will offer my admission to the one without the offer. People do that all the time. Also, it looks bad on stats if you offer admission to 25 people and only 7 accept your admission. Understand?
2. I suppose it could create a small amount of competition, but I doubt that. I could only see mentioning this to other professors if you already have an offer AND you are trying to get better packages from each institution.
I dunno... I would just stay silent... but thats me... Any other ideas guys?