It looks like UFC welterweight champion Georges St. Pierre will be making his next title defense in October against Johny Hendricks. So that means it's likely not going to happen in Canada. That comes straight from the mouth of Dana White (via twitter), who gave Joe Ferraro the news on UFC Central:
Currently the UFC has no official cards booked for October, but it had been expected by many that GSP/Hendricks would headline UFC 165, which takes place in Toronto on September 21st. With that out, and Dana White's recent comments about Lorenzo Fertitta wanting GSP to fight in Las Vegas, you'd have to guess that there will be a PPV (UFC 166 or UFC 167) held there in October and that's where the fight will land.
That's all speculation right now of course, but it does leave bring about another question - what will be the main event of UFC 165 now? Will Jon Jones defend his UFC light heavyweight title in Canada once again? The only other big option would probably be Cain Velasquez vs. Junior dos Santos 3 for the UFC heavyweight title, and that seems a little tight for time, so it could very well be Jones vs. Alexander Gustafsson in the Air Canada Centre.