Over the past few months, Google (NSDQ: GOOG) has been quietly testing its first mobile app ads with developers, including Backgrounds, Sega, Shazam and Urbanspoon, the company will announce this afternoon. The search giant’s approach to the mobile web has been fairly cautious, though it began fashioning its AdSense and AdWords system for mobile nearly two years ago. The tests have been limited to just those four developers, but Google is now opening the beta to a more developers who meet the company’s “specific criteria,” which includes a minimum of 100,000 daily pageviews, a willingness to go live with ads in four weeks. Developers also have to agree to a 3-month commitment. The AdSense for Apps is only available for Google’s Android and the iPhone. The site for developers is now live here.

The introduction of AdSense for Apps follows AdMob and others that already offer in-app advertising. The new program also fleshes out Google’s overall plan for mobile ads, which was revealed last February. Similar to its approach on the PC, Google has been focusing on two kinds of mobile ads: “contextual targeting,” and “Image Ads,” both of which which are variations on what they do on the desktop. In video promoting Google AdSense for Apps, Urbanspoon business development head Howard Steinberg, says the app will let the food guide target ads according to geography and keyword.

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