I just got this statement from a Google spokesperson:

Google is committed to finding new and better ways to get users the information they need while on the go, and to opening up new revenue opportunities for our partners. We are currently conducting a limited beta to test AdSense for mobile, a monetization product for mobile publishers. We will continue to evaluate the beta and will refine the product based on feedback from our users, publishers, and advertisers. We have nothing further to announce at this time.

Google wouldn’t say anything about the scope of the test, but offered to keep me in the loop on future updates.

I did manage to dig up a few more details, however.

According to information on Google’s AdWords Help Center, mobile ads are available in 13 countries: the United States, the United Kingdom, Japan, Germany, France, Italy, the Netherlands, Spain, China, Ireland, India, Russia and Australia. Advertisers can target their ads to specific countries. Google says the cost is similar to its standard PC Web ads.

The ads contain two lines of text, with 12 or 18 characters per line, depending on the language. Advertisers have the option to let people call their business directly from the ad. Advertisers pay only when someone clicks on the ad, or initiates a call from the ad. At right, what it might look like:

Google’s limited beta could be an important moment for the mobile Web. Assuming cell phone users respond to contextual advertising, the resulting revenue stream could compel media companies and retailers to invest more time and resources into building compelling mobile sites. It should also compel rivals Yahoo (YHOO) and Microsoft (MSFT) to ramp up their own mobile ad offerings.

If mobile ads are successful and create an ecosystem of mobile content, more people are likely to actually get online from their phones – and the price of Web-ready phones and service should drop.

Google CEO Eric Schmidt has already said it’s possible that cell phone ads could subsidize the cost of phone service – these might be the first steps toward that becoming a reality.