It was apparently too tempting to pass up. Yahoo started its own AdWords campaign on Google, advertising on the keyword "RSS" with a link that leads to an RSS resource page on its site. There, most of the links, as on much of yahoo's directory sections, are paid placements, suggesting that the campaign may be an effort at keyword arbitrage. In keyword arbitrage, Yahoo would pay for clicks with the expectation that the Yahoo advertisers will pay more once the original Google visitor arrives. Thanks to reader Steve Rubel for the scoop.