The Apple iPhone and Android devices have the U.S. smartphone market locked down according to a new study released Thursday by Nielsen.

Of all the smartphone owners in America, 48 percent own Android devices, while 32 percent use the Apple iPhone, though the ratio goes to 48 to 43 when looking only at people who bought their phones in the last 90 days, according to the study.

About 49.7 percent of mobile subscribers in America own smartphones as of February, which is a 38 percent increase in smartphone penetration over the last year, and of those, Android phones are the most-popular choice.

This growth is driven by increasing smartphone adoption, as more than two-thirds of those who acquired a new mobile device in the last three months chose a smartphone over a feature phone, Nielsen explains on its website.

But the iPhone only comes in a couple of models, with the iPhone 4S being the newest and the iPhone 5 release date being one of the most anticipated tech events on the horizon. As such, the level of smartphone market penetration that Apple has reached is astounding.

While Android has maintained an even keel over time, locking down 48 percent of current smartphone owners as well as 48 percent of those who purchased smartphones in the past three months, Apple has increased its market-share exclusively by moving in on the territory of lower-tier smartphone platforms.

For instance RIM Blackberry, which can claim 12 percent of current smartphone owners, was only the purchase of choice for 5 percent of those who bought their device in the last 90 days. Nielsen's reveal of this trend toward irrelevancy for the once-mighty RIM Blackberry comes on the same day that former Research In Motion chief executive Jim Balsillie stepped down from the company's board in the face of the news that the company lost $125 million in the fiscal fourth quarter.

All of the lost market-share for RIM Blackberry as smartphone owners upgrade is going to the Apple iPhone, as is the market-share for the 4 percent of buyers who adopt smartphones on other platforms like Windows Phone, Symbian and HP/Palm's WebOS.