GSP has been on his back for (I believe) three minutes in his last ~150 minutes of fighting: once against Kos, a minute against Fitch, and a minute against Alves after pulling his groin. If Kos and Fitch were incapable of keeping him there, I doubt that Shields would fare any better. That is also assuming Shields could get him there w/o being stuffed and/or reversed, or even more likely, put on his back prior to that.
Shields was great last night, but his style mirrors GSP's, except for the fact GSP is excellent at everything Shields is reasonably good at! All this being said, Shields is still a credible threat to GSP which is something the UFC still needs. I am interested in seeing GSP tested.