The points stated here somehow do make sense especially use of cookies and browsing pattern. It's almost fool-proof trying to cheat your way through.
I wouldn't think it's that difficult to trick them. There was an article on how to hack ppc with javascript. It was something like having 2 sites and they would some how trick the browser into loading a link from the second site using a popup under. The written said that they tried it with several sites and it worked everytime.