Android's rapid gains on the iPhone in ad impressions have come to a halt

For the second month in a row, two smartphone operating systems are in a dead heat

Source: Millennial Media

The extraordinary growth of Google's (GOOG) Android phones can be traced in six months of smartphone advertising data from Millennial Media, the largest independent mobile ad network.

In a series of pie charts, they show Android overtaking Research in Motion's (RIMM) BlackBerry between July and August in ad impressions, gaining rapidly on Apple's (AAPL) iPhone between June and September and finally coming even with iOS in October with a 37% share apiece.

But an interesting thing happens in Millennial's November report, released early Tuesday. It shows Android and iOS tied for the second month in a row, at 38% each.  After six months of breakneck gains, we have something that looks like equilibrium.

Apple is still the No. 1 smartphone manufacturer, as it has been for 14 months in a row, but there are signs that the iPhone is approaching saturation in the developer community. In a Millennial survey, Android was the No. 1 platform developers plan to support in 2011, with Microsoft's (MSFT) Windows Phone 7 and the iPad tied for second. The iPhone was relegated to fourth place, after the BlackBerry.

Below: Six months of Millennial pie charts.

Source: Millennial Media

[Follow Philip Elmer-DeWitt on Twitter @philiped]