Summer X Games was supposed to be in Montreal Canada this year, but because of lack of sponsors they decided to keep it in LA - that might be what you're thinking about. As far as Winter X, Aspens got that locked through 2012.
While Mammoth would be epic and most likely a better place for the games, it would be a bad move on ESPN's part because it's like five hours from LA. In my opinion, the X-Games should move around like the Olympics.