After lagging behind Apple’s iPhone for some time, Google’s Android platform is now tied for the lead spot in ad impressions on Millennial Media’s network, according to the company’s October Mobile Mix Report.

Both mobile platforms represent 37 percent each of Millennial’s smartphone OS impressions. Android jumped up 8 percentage points since last month’s Mobile Mix report, while the iPhone OS dropped 9 percentage points.

Millennial’s mobile ad network covers 73.5 million unique users, which gives it lots of data to work with when it comes to research.

Overall, smartphone impression shares increased by 3 percentage points since last month, and accounted for 61 percent of all smartphone, feature phone, and connected device shares.

Apple remains the leading device manufacturer on Millennial’s network with a 25 percent share of the top 15 manufacturers. Samsung jumped to the second spot thanks to a 3 percentage point gain, which the network attributes to the launch of its Vibrant Galaxy S phone as well as two other new devices. Motorola became the number three manufacturer with a 15 percent impression share.

Surprisingly, Research in Motion is seeing significant growth on Millennial’s network as well. The BlackBerry Curve displaced Motorola’s Droid as the network’s second most popular mobile device, and overall RIM had six devices in the network’s top 30 mobile phones. RIM’s ad requests have also increased 43 percent globally since last month.

Android’s growth isn’t surprising, but it will be interesting to see how long it continues its accelerated growth. I’m also not quite sure what’s at the heart of RIM’s growth, but I would wager that it may have to do with BlackBerry devices getting cheaper and becoming available as prepaid devices.

Mobile Mix October 2010 deviceOS