TechCrunch claims. Why would one highly profitable company dump such a large amount of duckets into another successful company's coffers? TechCrunch believes that it's part of a strategy to partner with Zynga Games in the forthcoming launch of something called "Google Games."
The service is said to use Google logins and will presumably run the Zynga titles popularized by Facebook, albeit with a different payment system (allegedly Google Checkout rather than PayPal). Though these claims are still unsubstantiated -- neither Google nor Zynga have officially confirmed the report -- a recent Google job listing for a "Project Management Leader, games" seems to lend the idea some credence.