I think GSP will win via split/unanimous decision.
Shields is good and will probably try to be akward by constantly shooting low like he does when people try to punch him. But while he shoots low I can see GSP being too strong for him by stuffing it or reversing him into an advantageous position.
While people say Shields stand up is weak, suprisingly he landed a few good shots on Martin Kampmann and Kampmann has a very good stand up (look at the Paulo Thiago fight) so I'm sure it's improving everytime; but not on GSP's level and probably never will be.
I think GSP will land some good combinations in the stand up(typical superman punch > leg kick) and also he has nasty clean hooks. I think he will take shields down and I also think he will be reveresed too because the ground battle will be a jiu-jitsu war.
I'm confident the stand up is GSP's and he may even knock sheilds down, but I'm not so confident about the ground area beause although GSP is a master, so is shields....