The main reason you have to place MMA so highly in America is because the numbers TUF drew when Kimbo Slice fought. Almost 7 million people tuned in to watch that fight, and it occurred during a baseball playoff game. Moreover, MMA's success on network television has been spectacular as well, with Strikeforce and EliteXC bringing in great numbers. And of course... there's the UFC's pay-per-view revenue, which of course is at an all time high.
Now, while the Kimbo fight trumped a baseball playoff game in ratings, and surely would have done the same to a playoff basketball game (not a football game, though), I still can't put MMA in front of them. There just aren't enough house hold names in the sport to do so right now, and that's what keeps me from doing that. Also, the UFC attendance records don't come close to matches a lot of the top sports in America, either.
As far as how popular I think this sport will end up being.... I'm not so sure. I think it'll continue to do well, and I do think it's on the verge of competing with golf as the most popular non-team sport in America, but I don't think the UFC will ever get to the point where every pay-per-view gets over 1,000,000 buys, and the television ratings average 6 million viewers every show they air. I do believe MMA is here to stay though, and will remain popular, but at the same time... I don't think they're far away from peaking. But what's good about that is that I think they'll peak and stay there, rather than go down hard soon afterward, like a lot of things do when they peak at something.