Android and iPhone developers looking for an extra source of revenue will soon have an advertising option, sponsored by Google.

Google's AdSense program--in which it sells and distributes ads to third-party publishers--is coming to mobile devices as a beta program, the company announced Wednesday. A small group of developers have been testing this program but now anyone can apply, said Susan Wojcicki, vice president of product management, in a blog post.

Mobile apps are one of the hottest topics in software development these days, as Apple's App Store continues to grow and other developers roll out their own app stores in response. Plenty of money is being made simply on the sales of the applications themselves, but additional revenue streams are starting to emerge, such as Apple's addition of in-app purchases with the release of iPhone OS 3.0.

Google, naturally, would like to get in on the action. There are other companies offering this kind of service, such as AdMob's strong presence in iPhone apps, but Google is an advertising freight train.

Still, AdSense growth has slowed this year amid a plunge in advertising spending around the world, and moving into mobile provides additional room for the company to grow. Last year Google offered AdSense for games, another step outside its usual comfort zone.

There are a few catches if you want to participate in the beta. Your application must generate 100,000 page views a day, and it must be free. Google is taking applications here.