UFC President Dana White announced the news at the post-fight press conference.
Diaz was originally slated to face St. Pierre for the championship at tonight's event, but due to a series of missed media events he was inserted against Penn.
Now, he gets his opportunity to dethrone St. Pierre.
During the fight announcement, White stated that they're looking at Super Bowl weekend of next year for the championship mega-fight.
Welterweight contender Carlos Condit stepped aside to make Diaz and St. Pierre happen in the first quarter of 2012.
With the move, Condit informed White that he wants to return to action on the same card as St. Pierre and Diaz.