• by Mark Walsh  @markfwal, January 18, 2012

Last year ended with smartphone penetration in the U.S. reaching 46%, according to the latest data from Nielsen. That’s almost at the halfway mark predicted by the media research firm before the start of 2011 and up from 30% in the fourth quarter of 2010. Among people who got a new mobile device in the last three months of 2011, 60% bought a smartphone.

Helping to drive adoption in recent months has been the iPhone 4S, launched in October.

The popularity of the latest Apple handset is underscored by the Nielsen research, which shows that among people who bought devices in the fourth quarter, 44.5% in December said they got an iPhone compared to only 25.1% in October. Among new iPhone acquirers, 57% got the 4S.

Conversely, Android’s share among people who bought smartphones in the last three months of 2011 dropped from 61.6% to 46.9%. Struggling BlackBerry dropped from 7.7% to 4.5%.

Even so, Android managed to strengthen its position as the most pervasive mobile operating system among all smartphone users in the fourth quarter. All told, 46.3%. of smartphone owners surveyed owned an Android device, up from 42.8% in the prior quarter. Apple’s iOS platform increased its overall share slightly to 30% from 28.3% in the third quarter.

By contrast, BlackBerry slipped to a 14.9% share overall in the fourth quarter from 17.8% in the prior quarter, while Microsoft’s Windows Mobile and Windows Phone platforms dropped to 4.9% from 6.1%. Microsoft, of course, has launched an ambitious effort to reverse its continuing slide in the smartphone market with the new lineup of Lumia devices developed in partnership with Nokia. 

How well consumers respond to the Nokia/Windows Phone handsets will be unclear until sales results come in later this year. But Wal-Mart’s decision to offer the entry-level Lumia 710 for free with a two-year contract suggests that pricing will not necessarily be a barrier to Microsoft making headway in the smartphone battle in 2012.