You know all those spammy sites that are only made to earn Adsense revenue, that are clogging up the engines? Here is a piece about it with links to some good forum discussions from Chris Boggs over at Search Engine Roundtable.

Publishers and Arbitrage: Sucking the Life Out of Search Results? “AdSense became a popular Internet Marketing platform through its two-pronged model of selling keyword based ads and paying outside websites (known as publishers) to host them. Since publishers have found out this is a proven way to generate income, many sites have been developed solely for the purpose of hosting AdSense or Yahoo Publisher network ads. Publishers have discovered that by paying to drive traffic to the site, often through AdWords and AdSense accounts of their own, they can turn a profit when visitors click on the hosted advertisements.”

See the example in the link above about how the arbitrage numbers and ROI work, in this game of chance and sometimes profit.