Mobile ad firm Millennial Media says that mobile connected devices like the iPad and Sony PSP displayed more than 20 percent of the ad network’s ad impressions in March. It’s a trend the company says could have a far-reaching impact on mobile advertising as such devices grow more popular.

Millennial also found that global Android ad requests grew 72 percent month-over-month, and global RIM ad requests grew 25 percent in March.

The findings, part of the new Millennial Media Mobile Mix report, complement the company’s monthly Scorecard for Mobile Advertising Reach and Targeting (SMART) report.  

Other findings from Mobile Mix include a breakdown of U.S. device impressions. Smartphones accounted for 45 percent of all impressions, followed by feature phones (34 percent) and connected devices (21 percent). 

Apple’s OS remains the leading OS on the network, with 70 percent of U.S. smartphone impressions. Apple’s OS has maintained the leading OS position since Millennial began reporting the network’s OS mix in August 2009.

RIM remained the second largest U.S. OS for the eighth consecutive month with a 14 percent share of impressions and four devices in the Top 20 Mobile Phones list. 

While Apple represented 40 percent of network impressions in March, manufacturers supporting Android-enabled devices represented over 50 percent of the Top 15 Manufacturers. 

Smartphones accounted for 40 percent of the Top 20 Mobile Phones, with the BlackBerry Tour, Palm Pixi and HTC’s MyTouch Magic new to the list.