Of course, this is just another attempt to fend off Microsoft's ongoing Yahoo! takeover bid. On February 1, Microsoft bid $44.6bn for the world's second largest search engine, and the Redmond software giant is now threatening a proxy fight if Jerry Yang and company don't succumb. When Yahoo! announced its Google ad test, Microsoft said that any "definitive agreement" between Yahoo! and Google "would make the market far less competitive." And there's little doubt that such a pact would face scrunity from regulators.