On September 18, 2007, AdSense for Mobile was launched, before the smartphone era and when everyone had a flip phone. Since, then, mobile technology has advanced a lot and also increasing number of consumers viewing content from "smarter" mobile devices. Now, Google migrated all its mobile ad unit sizes, including the mobile banner ad unit, into the core product to make it easier for publishers to use AdSense to monetize mobile web pages.
With that, now all mobile ad sizes, including the 320x50, will be available through AdSense for content.
The new AdSense ad code automatically formats the ads for the device.
"We will continue to support high-end ad requests from our AdSense for mobile content product until May 1, 2012," posted AdSense team.
Adding, "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," the team said.
Note that publishers with mobile websites built for WAP browsers should continue to monetize using AdSense for mobile content.
For more information about AdSense or to learn more about how this transition may impact you, visit AdSense Help Center.