I like the dresses madametj suggested...simply because at weddings, it is hard to look really "nice" if you are large chested and hanging out of your dress (and a D cup will be hanging out of those dresses...not enough material). While all of those dresses are beautiful, I think you will end up being distracted most of the evening by your, um, assets, attempting to slip out from the bra, or out of the v-necks, or popping out from the strapless dress. Even if her dresses are out of your price range, you might want to consider something a little more covered up. You want to come across as sophisticated and elegant...perhaps a bit older?
I would suggest a ball skirt and a DRESSY t-shirt (the kind that are meant to be worn under cardigans, suits, etc...not the athletic kind!). If you wore that, you could easily get away with cute flats (ballet slippers) or sandals. It's cute and young and fresh...but still elegant and simple. People will be looking at you because you are so pretty, and not becaus you are distracting. You could also find someting appropriate if you went into JCPenneys (every town has one of those!) and went back to their adult dress selection.
If you can avoid dresses that require strapless bras, do so. I am very small chested (a full A) and they don't stay on girls my size either. They just aren't comfortable or secure, and they cost a fortune to get a decent one.
Be a light unto the world...not a bug zapper.