After months of testing, Google is publicly launching the beta version of its AdSense for Games software. The product represents the company’s first push into a fast-growing new market — the ability to insert ads into games, either as they are being played or just after they finish.

The plan is pretty much what I reported in July, when word surfaced that the search giant was testing the ads. Its decision to enter the market with a public beta test is a big deal because it means that the game advertising market is becoming lucrative enough to attract an industry giant.

“It’s a huge stamp of approval for a major company to come in and say they believe in the online games ad market,” said Jameson Hsu, chief executive of Mochi Media, a game ad network operator which is partnering with Google. “It should awaken all of the major media companies.”

Google is initially targeting the sweet spot for its technology: games based in Adobe’s Flash platform and which run in a web browser with no download. These are free casual games that are like snacks compared to the full course meals of hardcore console or disk-based PC games.

The Flash market itself is a fast-growing segment, with roughly 200 million games played each month for a total of billions of minutes. It’s a good way to reach younger people or those who are hard to reach because they don’t watch TV commercials anymore.

Christian Oestlien, senior product manager at Google, said that the company has been testing the in-game ads for months with partners such as Playfish, a maker of casual games such as “Who Has the Biggest Brain?” on Facebook. At the end of a game session, the Google software will roll a 30-second video that includes a game character saying that the free game was brought to you by a sponsor.

Oestlien said that Google’s advertisers can use the software to insert ads into games or videos for YouTube, making the ads more versatile. Developers of games can use Flash software development kits to designate the points in a game that make an “ad request.”

“We’ve tried to integrate this in a user-friendly manner,” Oestlien said.

Advertisers participating in the trials so far include eSurance, Sprint, and Sony Pictures. Other partners include Demand Media and Boonty. Partners such as Mochi Media can help Google by supplying highly trafficked games that can receive ads from Google’s network of advertisers. Google’s main advantage in this business is its large ad sales force, which may generate more advertising inventory than any single player can handle. That’s why it’s important for Google’s ads to be able to run on a variety of sites.

Besides Playfish, other game publishers that will incorporate AdSense for Games technology include Zynga, the hot social gaming company, and Konami, one of Japan’s biggest game companies. The latter will publish a line of Flash titles — including “Track and Field,” “Frogger,” and “Dance Dance Revolution” — with the Google in-game ads.

Full told, more than two dozen games will use Google AdSense for Game at the outset. Oestlien said that the games have millions of users and can create several hundred million ad impressions over time on sites such as MySpace or Facebook.

Sebastien de Halleux, chief operating officer at Playfish, said that Google’s technology is a good match for his company’s social games and that the pilot went well. Google takes a cut from the ads and the rest goes to the game publishers, distributors or developers.

Oestlien declined to say whether Google would expand beyond the Flash casual gaming segment. But Google’s entry into Flash web games is likely just the first step. After this, you can expect to see the company move into