A new survey out of Nielsen today says nearly one in three of all smartphone users in the United States have an iPhone, or 32.1 percent. For comparison, 48 percent opted for an Android device, or nearly one in two. As for recent acquirers who got their smartphone within the last three months, 43 percent bought an iPhone versus 48 percent for Android.

Together, the two platforms pretty much own the entire US smartphone market. More important than this, in my opinion, is the overall U.S. smartphone penetration rate which approached the psychologically important 50 percent milestone during the month of February. How is smartphone penetration important?

Compared to February of last year, only 36 percent of cell phone buyers opted for a smartphone versus 49.7 percent in February 2012. That’s a 38 percent annual increase, mind you. Furthermore, more than two-thirds of those who acquired a new mobile device in the last three months chose a smartphone over a feature phone.

First of all, this means there are still more folks out there who haven’t upgraded yet from a feature phone to a smartphone, representing an ample opportunity for both Apple and Google to grow their respective sales.

This means the mobile race is not over yet.

As Microsoft re-enters the market with Windows Phone and Research In Motion continues on a downfall spiral (Blackberry owners represented just 11.6 percent of the US smartphone buyers), it’ll be interesting seeing whether the Windows maker can break the iOS-Android duopoly and establish itself as a credible No. 3.

AT&T on its part is keeping its fingers crossed for an upcoming launch of Nokia’s Lumia 900 that the carrier said will top the iPhone, though we find this hard to believe.

Ironically, Apple’s success in the smartphone race is benefiting Google as the search giant collects mobile advertising revenue from adverts injected in iOS mobile search results, maps and even third-party apps that use Google’s AdMob and AdSense technologies.

In fact, court documents suggest that iOS earns Google four times the revenue of Android.

Looking at the big picture, and this comes from the mouth of Apple’s lawyer, once people choose a smartphone, they remain loyal to that platform and are likely to make their future purchases in that environment.

So yeah, stickiness matter and with one in three smartphones sold in the U.S. being an iPhone, I think Apple is well poised to continue to give Android a run for its money while leading the industry in terms of innovation.

What did you think of those numbers? Would you say, based on your everyday experience, that even more people have an iPhone than the Nielsen survey would have us believe?