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Part of that success for mobile ads could relate to the novelty of the format, Strother suggested. As people grow more accustomed to seeing ads on mobile Web pages, clickthroughs will decline, the argument goes.

Nevertheless, Apple's (NASDAQ: AAPL) iPhone has shown that with a decent screen size and intuitive navigation, U.S. consumers will use their phones for activities other than talk and text messaging. Google is hoping that efforts such as its own Android initiative will lead to a new generation of handheld devices that help the mobile Web live up to its promise.

While "some of the bigger brands have made some serious commitments to mobile," Strather said that the tendency among advertisers is to make mobile a microcosm of an aggregate digital budget, or to treat mobile advertising as an experimental expense. "Very few companies on the advertising side have made mobile a standalone item on a line-item budget," Strother told InternetNews.com.

The company is trying to keep file sizes small, so that the ads do not unduly slow the load times of mobile Web pages, Agarwal said. Slow speeds have been a common complaint about the experience of browsing the Web on a mobile device.

Google also said it will only show one display ad per page.