I don't see this ever happening.
GSP fights at Welterweight (155 lbs to 170 lbs). GSP does not have to lose much weight to fight at this weight class; not much at all.
Silva fights at Middleweight (170 lbs to 185 lbs) and is at the top of this weight class and must lose weight to fight AND had to lose a little just to fight in the Light Heavyweight (185 lbs to 205 lbs); though not much.
So say Silva wanted to fight at Welterweight he would have to lose 40 lbs or more to do so; that is way too much weight to lose. I don't know many fighters that can do this and still be strong with the exception of Sean Sherk. And since we already knew and it was proven with testing, Sherk only has to stop taking his supplement "S" to help this cause. And why would GSP want to fight at the bottom of the Middle Weight class when he is at his best right where he is at the top of the Welterweight class?
Again, don't see it happening thought it would be a good fight.
"IF I COME ... I'M BRINGING THE PAIN WITH ME"