Arbitrage works like this: you sign up for a program that pays X per click. You buy clicks from a network for Y per click. Your profit is the difference between the price of X and Y taking into consideration the "fraudulent click filtering" % of your sponsor (is "shave" a harsh word?

Let's settle for "scrub"


) Anyway, if the price you buy at is LOW enough, you still make $$$ even with tight scrubbing.
Twitter clicks are the latest focus of arbitrage money making activities. Check this case study out:
http://www.shoemoney.com/2009/07/13/...itter-traffic/
Adsense/Adword arbitrage used to be nice until Google poured piss on it by tightening their Quality Score system (Adwords), this made the system unprofitable.