Other than a freak upset like we saw with Serra, Diaz has no chance in hell with GSP. In fact, Diaz would not get through the UFC's top welterweights like Fitch, BJ, Koschek, etc.
Nonetheless, GSP has completely cleaned out the 170lbs division and all that we have left are overhyped cans like Diaz.
I know everyone, myself included, wants to see GSP demolish his opponents in spectacular fashion. But, winning is more important that flashy finishes. GSP knows this. And, that is why we will see a 5 round bump & grind decision win for GSP.