Ummmmmm...........No. That fight will in all likely hood will not happen. Silva at 185 is already skinny as shit, theres no way hes going to drop down to 170.
St. Pierre would have to move up to middleweight, but if he does it will only be to fight him and only him, because 4 of his training partners are in the 185 class(Patrick Cote, Nate Marquardt, David Loiseau, Jason MacDonald) . If he does defeat silva and wins the belt, he will in most likely hood have to fight one his training partners.
Anyways, hes going to stay at welterweight and completely dominate that class.