Speaking of GSP: he was pretty vocal about PED use and also stated that anyone could get away with it based on the testing they were doing at the time and anyone who did get caught was lazy and/or dumb. And even with the recently "improved" testing methods, I imagine one could still get away with PED use if they cycled right and took the right stuff.
So... why did Silva get lazy and get popped? He just figure he (and just about everyone else) got away with it for so long that he couldn't get caught?
One thing I think the Diaz bros have mostly right is that the majority of the UFC is juiced in one way or another.