My personal antagonism towards Diaz aside, I don't see how he wins this fight. St. Pierre will be too big, too fast, and better in every area. Diaz has unorthodox striking that may compare offensively to St. Pierre's, but St. Pierre's defensive game is much better than Nick's.
I can see St. Pierre kicking the hell out of Diaz's legs while the fight remains upright, and then take Diaz down when he brings the pressure. There's no way Diaz is submitting St. Pierre, his jiu jitsu is solid, yes, but St. Pierre may have som of the best submission defense in the UFC, if not the best, period.
St. Pierre via UD (50-44, 50-44, 50-43)