Android accounts for over 50% of the mobile ad impressions on the Millenial Media mobile network

"Today, we released our August Mobile Mix Report, and for the first time, we combined smartphones and connected devices when breaking down impressions on our network by operating system. When looking at both connected devices and smartphones, we saw that Android had a 54 percent share and iOS had a 28 percent share."

Source:  Blog post from Millennial Media, 23rd September 2011