Mobile ad network Millennial Media put out a new "Mobile Mix" report, reflecting acitivity and devices on its network of sites and publishers. The company said that smartphones were 45% of the devices generating impressions on its network. Currently smartphones constitute somewhere between 20% and 24% (on the high end of estimates) of the US handset market. 

The iPhone was the single top device but Millennial said that multiple Android handsets were also gaining. The company also said that it saw the Palm Pixi show up in its logs for the first time: 

• Apple’s impression share increased by 1.5% to remain the number one device manufacturer on our network, a six-month trend that has continued since September 2009
• Smartphones accounted for 40% of the Top 20 Mobile Phones in March with the Blackberry Tour, Palm Pixi and HTC’s MyTouch Magic not previously seen in February’s Top 20 list
• While Apple represented 40% of impressions in March, manufacturers supporting Android enabled devices, including: Samsung, Motorola, LG, HTC, Kyocera, HUAWEI, Sony Ericsson, and ZTE, represented over 50% of the Top 15 Manufacturers on our network

Millennial also said that the iPad has appeared on its radar: "Apple’s iPad impressions increased 713% during the first full week that the device was in-market (4/5-4/12/10)." It promised a more complete analysis of the iPad next month.

Growing from zero of course the iPad numbers on a percentage basis will look very large. Over time it will be interesting to see how big a contributor to "mobile impressions" and "mobile search" the iPad is. Google is clearly treating the iPad as a "high end mobile device." But it really blurs the distinction between mobile and netbooks or laptops in terms of how clicks and impressions should be accounted for.