While it’s no secret that smartphones play a vital role across the contemporary digital advertising landscape, a new report from Millennial Media reveals that smartphones now account for nearly half of mobile ad impressions in the United States today.

Millennial Media – a mobile ad network with an expansive reach that covers better than 80% of all US mobile subscribers – also delineated ad serves on other devices beyond mobile phones. This is providing mobile marketers with a unique opportunity to learn about ad serves on everything from iPads to mobile gaming devices connected over WiFi.

The break down shows that, all told, mobile devices capture a significant chunk of all ad impressions, with iPad impressions in particular exploding some 713% in the first full week of the Apple tablet’s release. iPhones, however, continue to lead the pack and “dominate impressions” in the smartphone category – a category that claims 45% of the mobile ad impression pie.

In addition to the sizable chunk of ad impressions captured by smartphones, feature phones are responsible for another 34% and connected devices round out the rest with 21%

In the big picture, Apple is clearly becoming a bigger player in the mobile ad space than many realize. But as Millennial Media illustrates, don’t count out others – especially the Android, whic has surprisingly gained a good deal of ground against Apple, likely the result of “high customer satisfaction” and its availability from a variety of manufacturers and carriers.