Handsets and other mobile devices offer "huge opportunity" for online search giant to expand its ad-placement program Google has begun inviting mobile Web site developers to display Google ads on their sites as part of a limited beta test.The offer extends to the mobile environment Google’s AdSense program, which lets Web developers earn revenue by placing advertisements on their sites. Google runs the back-end network that places ads on the sites relevant to content. Site owners earn revenue when visitors click on the ads.Sites must be written in one of three mobile markup languages: WML, XHTML, and cHTML (compact HTML) in order to use AdSense for mobile, according to a Google AdSense for mobile help page. That’s because Google’s crawlers must be able to read the page in those languages to determine page content and serve up relevant ads. A blogger at Self Made Minds said that he received an e-mail invitation from Google on Thursday night to test AdSense for mobile.In a statement, Google confirmed that it is conducting a limited beta to test AdSense for mobile. The company plans to evaluate the beta and refine the product based on feedback from users, it said.Late last year, Google began delivering advertisements with its mobile search results. Online giants such as Google are increasingly interested in the potential revenue stream from mobile users. “Mobile advertising is a huge opportunity for us, starting with the basic premise that there are something like 3 billion or so handsets in the world,” said Dilip Venkatachari, director of product management responsible for mobile monetization efforts at Google, in a recent interview. That compares to just around 1 billion PC users on the planet, making for an even larger target market. Software Development