Red John is Michael Kirkland. He is an identical twin of Bob Kirkland and Patrick has no idea that he met him before. He is the one that appears in the scene where Robert Kirkland kills Jason in the hospital: http://www.youtube.com/watch?v=eMFbgTz50EM

Bob Kirkland would never kill Jason without first asking him if he knew who Red John is. This has to be Michael in that scene and then quickly goes away when he sees Patrick so that he doesn't figure out he is a different person.

Also, at the promo it appears that Red John is someone that Jane didn't expect because he claims that the game is over and that he won. Moreover, his voice sounds similar to Kirkland's.

This is my theory. It seems very fishy that they would have someone with an identical twin without any reason.