• by Mark Walsh  @markfwal, June 30, 2011

Android remains the dominant smartphone platform, but its growth has leveled off in recent months, according to the latest data from Nielsen. As of May, 38% of U.S. smartphone owners had an Android device compared to 27% who owned an iPhone and 21% who had a BlackBerry phone. Another 9% had a Windows Mobile device.

But among people buying smartphones in the last three months ending in May, Android's share has held steady at 27%, while the iPhone's has increased from 10% to 17%. That finding coincides with other recent reports suggesting uptake of the Google mobile platform is slowing as a result of Apple launching the iPhone on a second carrier-Verizon Wireless-earlier this year.

Needham analyst Charlie Wolf issued a research note using IDC data to show Android's U.S. market share peaked in March, dipping to 49.5% from 52.4%. He suggested that trend could accelerate with the expected launch of the iPhone 5 on the Verizon and AT&T networks. The upcoming iPhone could also launch on the Sprint and T-Mobile networks.

Separately, BTIG Research last week released a report saying the iPhone is outselling Android devices in the majority of U.S. AT&T and Verizon stores. Verizon said in April it had sold 2.2 million Verizon iPhones in its first two months on the network. And Apple reported selling 18.7 million iPhones during its fiscal second quarter ending in March.

The Nielsen data comes only a couple of days after Google mobile chief Andy Rubin announced via Twitter than the company is now activating 500,000 Android devices a day globally. That's up from 300,000 in December and 400,000 in May. Apple CEO Steve Jobs has accused Google of inflating activation numbers by counting updates and re-installs. Apple said in January it was activating 360,000 iOS devices a day. 

One thing is clear: Competition between the two tech giants in mobile is helping driving smartphone adoption. Overall, 55% of those who bought a new handset in the last three months purchased a smartphone, up from 34% a year ago.

Microsoft, however, does not appear to be benefiting yet from the trend. Despite the launch of Windows Phone 7 late last year, the proportion of people buying smartphones running the Microsoft platform in the last three months is stuck at just 1%.