The mobile ad industry is still in it's infancy -- mainly because mobile browsing is as well. Companies like Google look at emerging markets with wide eyes, and that's why it's not surprising that Google is starting to do more testing with mobile ads.
Companies that use Google's AdWords to advertise online now have the option of displaying a banner ad on cell phones instead of simple text. Google includes a text line under each banner identifying it as an advertisement. Clicking on the ad opens a mobile Web page for the advertisement. -- PC World
Until now, Google only offered advertisers the ability to purchase text links for mobile devices -- but that has changed. Now advertisers can publish banner ads too. Since the mobile browsing experience is becoming more like "the real internet", ads that were traditionally used on webpages can work just fine on this generation of mobile browsers.