Well Rory Mac tossed Nate around at 170. So sure you are right.
But here is another thing. People really must not think much of GSP. We are talking the #1 and #2 best fighters in the world. How is that comparible to GSP and Nate Diaz?
People are protecting the #2 fighter in the world as if he is some decent fighter. No, he is easily the best 170 in the world. And stylistically he matches up well with Anderson's weakness. Anderson cutting would hinder his strength. People tend to see Anderson as his 205lb thick self. Not, he will be cutting a lot of weight and losing a lot of muscle. It isn't little GSP vs. big chunky Anderson. It would be a more filled out GSP vs. a more slender and drawn out Anderson.
Edit: Also you are crazy if you don't think Nate Diaz would take that fight. He definatly would. Only way he wouldn't is if he thought his brother should get it and didn't want to get in his way. Nate Diaz wouldn't decline any fight.
There aren't exactly any fighters in the LW division that are top 5 pound for pound, so Nate was the closest example I could come up with. And the reason I used Nate, was to show how ridiculous it is for people to give a fighter shit for not going up to fight someone who's in another weight class.
Now, although GSP is one of my favorite fighters, and one of the top 3 fighters pound for pound on this planet, we are talking about Anderson Silva. I'm not even a big fan of Anderson's but I can't deny his greatness inside the octagon. Even though he cuts to lets say 180 pound catch weight, he'll still have a massive strength advantage in my opinion. Obviously strength isn't everything but when you are talking about two of the pound for pound best, every advantage makes a huge difference.
The bottom line is that I am indifferent to this fight. But if it happens, i know i'll get pumped, but I won't lose any sleep if it doesn't happen either.