They have worked so hard to get MMA sanctioned that if it began to become fixed, there would be some huge financial and legal repercussions if it were discovered. So much that it would probably hardly be worth it.
However... I could see some of the pre-fight hype become more-so scripted, and I think in some organizations, some behavior is certainly influenced. Like in that interview (I forgot who it was, I read about it here) when the guy said the Pride guys tried to get him to make some bigoted remarks against Japan. I think this is a much more real possibility.