Google AdSense for mobile was put in place while people were still using flip phones, in order to allow companies to monetize their mobile content. With the advent of smartphone popularity, Google has just made it easier for vendors to use AdSense mobile, by migrating all mobile ad unit sizes, as well as the mobile banner ad unit, into the core product. The new AdSense code automatically formats the ads for the the particular device.
Google states it will also continue to support high-end ad requests from AdSense mobile content until May 1, 2012. In the meantime, Google states, "we strongly encourage publishers who have designed mobile web pages for high-end devices to use the new AdSense ad code to avoid disruptions to service. Note that publishers with mobile websites built for WAP browsers should continue to monetize using AdSense for mobile content.
Google notes that it continues to be committed to helping our AdSense publishers monetize their content as the mobile ecosystem evolves.
For more information about the AdSense transition, visit Google's AdSense Help Center.