I'm not one to cry wolf, or cry nipple as it were, but in all honesty, I'm just not sure of what I'm seeing here. It really looks like a Gisele Bundchen nipple slip, but it could just be some weird shadow (or wishful thinking). The top of her dress is see-through, though, so I think I'm right.
Update: While I don't need the validation of others to make me feel better about myself, it's nice to know that I'm not going crazy. Well, at least not crazier than usual. TMZ spotted the Gisele Bundchen nipple slip too, so now it's official (not that it wasn't official before).