The fight between Demian Maia and Nate Marquardt this weekend is ostensibly for the number one contender position at middleweight, but in reality this is a one-sided contender's match. If Maia wins he will immediately be groomed for a shot after Henderson, but if Nate wins, I have significant doubts he will get a shot any time soon.
If Nate wins, Dan Henderson will get the next shot, probably in November or December. Nate would probably have to fight again inbetween. After that fight, Anderson will probably be much more interested in "money fights" against top light heavyweights and heavyweights, and Nate will probably have to fight yet again. The problem is that when you constantly fight top contenders, all but the most elite are bound to lose, and if Nate loses again he can wave his title hopes goodbye for another few years.
Unfortunately for Nate Marquardt, he got his title shot before he really hit his stride as a fighter. I still believe Anderson would beat him, but it would be a closer fight now. Even if he keeps winning spectacularly, the odds of him getting another fight with Anderson are low. He will probably have to bide his time until either Anderson drops the title, or a contender for Anderson gets injured and they need a replacement. Such are the breaks in an industry that lives and dies on pay per view.