Mobile ad network Millennial Media says its latest figures show that this month, for the first time, Android tied with Apple’s iOS as the largest smartphone OS on the Millennial network, with an 8% increase month-over-month and 37% impression share.

Android ad requests grew 65% month-over-month, and since January, Android has now grown 2182%. The iPod touch was included on the list of top mobile devices for the first time, and was number three (ahead of phones like the Motorola Droid) .

RIM ad requests grew 43% month-over-month, and RIM devices represented six of the Top 30 mobile phones (with the BlackBerry Curve being number two overall). iPad requests increased 112% month-over-month  .

Motorola became the number three manufacturer and held a 15% impression share. Two new Motorola devices — the Droid 2 and the Droid X — both made their entrance into the Top 30 Mobile Device list. The top 4 countries globally (in terms of impressions) were the US, UK, Canada and South Korea.