Nielsen has released some smarpthone market share figures for March. The metrics firm says that 50.5% of US mobile phone owners now carry smartphones. The data show the following breakdown by operating system:

• Android OS -- 48.5%
• iOS -- 32.0%
• Blackberry -- 11.6%
• Windows Mobile -- 4.1%
• Windows Phone -- 1.7%
• Other -- 2.1%

Here's comScore's March data for comparison purposes:

• Google -- 51.0%
• Apple -- 30.7%
• RIM -- 12.3%
• Microsoft -- 3.9%
• Symbian -- 1.4%

Comscore puts smartphone penetration at 45.2% in the US by comparison. 

There's a meaningful difference in the numbers in terms of Android and iOS market share percentages in both data sets. In addition Nielsen shows 5.8% penetration of Windows, while comScore shows 3.9%. That's also a fairly significant difference. 

They both agree however that the Windows Phone platform so far hasn't moved the needle and Microsoft's overall share of the smartphone market is declining.