After years of anticipation, Google is currently sending out invitation emails to some publishers for an AdSense for Mobile beta test. Google's invitation reads:
"As part of our efforts to develop new and improved AdSense products for our partners, we will begin a limited beta test for AdSense for mobile. AdSense for mobile allows publishers to monetize their mobile websites through the placement of targeted text ads."
Google already has an extensive FAQ page for publishers looking to create mobile ads for AdSense.