Research In Motion and Apple are duking it out for the top spot in the U.S. smartphone market, according to the "Global Smartphone Report" released yesterday by Nielsen.

Among the 13,243 smartphone owners polled by Nielsen, Apple's share has grown steadily over the past year, with the iPhone grabbing 28 percent of them all in the third quarter. On the flip side, RIM has seen its share for the BlackBerry drop to 30 percent for the same period. Those numbers include all people in Nielsen's panel who own smartphones.

The difference in demand between the two platforms also breaks down by age, according to Nielsen's numbers. Apple has the highest number of smartphone users under 44, while RIM has captured the most customers 45 and older.

In terms of sheer momentum, Android is the one heating up the market. Though still in third place among all of those surveyed who own a smartphone, Google's mobile OS has grown from a 4 percent share in the fourth quarter of 2009 to a 19 percent share in the quarter just ended, according to Nielsen. Among customers who picked up a smartphone over the last six months, Android did take the top spot during the quarter, said Nielsen, with BlackBerry and the iPhone battling for second place.

Other recent reports also point to Android's growing popularity. Another Nielsen report, from early October, found that 32 percent of the smartphones purchased from January to August were powered by Android, compared with 25 percent for Apple and 26 percent for RIM. And reports this week from NPD Group and Canalys showed that among people who bought smartphones just in the third quarter, around 44 percent of them opted for an Android device, leaving Apple and RIM with figures hovering in the mid-20 percent range.

Among other mobile operating systems, Microsoft's Windows Mobile has declined in demand over the past year and is now hanging onto a 15 percent slice of the market. The company is hoping to recapture some of that lost share with its new Windows Phone 7.

The Palm OS, now owned by HP, has also shed share over the past year. And Nokia's Symbian OS, though still the worldwide smartphone leader, has failed to take hold in the United States and has hung onto the same small share of the U.S. market since last year.

Smartphones in general continued to carve out a bigger chunk of the mobile phone market. As of the third quarter, 28 percent of all U.S. mobile phone subscribers own smartphones. Among consumers who bought a new mobile phone over the past six months, 41 percent of them chose a smartphone over a standard feature phone, up from 35 percent in the second quarter.