Read this and weep, Android, BlackBerry, and AT&T (NYSE: T) fans. The January launch of the Apple (Nasdaq: AAPL) iPhone on Verizon (NYSE: VZ) appears to have turned the tide between iPhone and Android smartphones in the United States.

The AT&T iPhone has been notorious for dropped calls and, in some densely populated areas such as San Francisco and New York, often no signal at all. A March study by ChangeWave Research reinforced the popular view that the dropped calls were AT&T's fault. Many observers wondered whether the dropped-call issue was a reason Android smartphones began outgrowing iPhones in early 2010. Nielsen's latest survey of U.S. smartphone purchases suggests that it was, and now Verizon has come to the rescue.

Nielsen's data showed that Android still outsells Apple, but Android's share of sales has stagnated at 27% while the iPhone has surged ahead to 17%, up from 10% in February. Much of Apple's gain seems to be at the expense of Research In Motion's (Nasdaq: RIMM) BlackBerry.

OS Share, U.S. Smartphones Acquired in the Past Three Months

Smartphone OS

Feb. 2010

May 2010

Aug. 2010

Nov. 2010

Feb. 2011

May 2011

RIM

10%

12%

9%

9%

11%

6%

iPhone

8%

7%

12%

11%

10%

17%

Android

7%

9%

15%

21%

27%

27%

Source: Nielsen.com.

A market worth fighting over
Another notable development is that mobile-phone share is shifting to smartphones. According to Nielsen's May survey, 38% of U.S. mobile consumers now own smartphones, with 55% of the new handsets purchased from March through May being a smartphone instead of a feature phone. That's up from 34% in the year-earlier period.

Foolish takeaway
Nielsen's survey bodes well for both Apple and Verizon, and it's a negative for AT&T and Research In Motion -- as if we didn't already know about RIM's woes. Even the outlook for Android phones is less robust than if they had continued to gain share. Stagnation in the Android phone market could slow growth for smartphone makers such as Motorola Mobility (NYSE: MMI), Samsung, and HTC.

There are potential spillover effects into the tablet market, too. The resurgent strength of iOS in smartphones could help the iPad continue its dominance, with negative implications for tablet makers such as Motorola Mobility, RIM, Samsung, Dell (Nasdaq: DELL), Hewlett-Packard (NYSE: HPQ), and, soon, Sony.

What do you think: Will iOS continue regaining share? Will Android continue to tread water? An easy way to stay on top of market developments is The Motley Fool's free new My Watchlist feature. You can get up-to-date news and analysis by adding these stocks to your Watchlist now:

Fool contributor Cindy Johnson owns no shares of any stock mentioned in this story. The Motley Fool owns shares of Apple. Motley Fool newsletter services have recommended buying shares of Apple and AT&T and creating a bull call spread position in Apple. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.