According to the market researchers at Nielsen, it's still a two-horse race in the US smartphone market. But thanks to a massive splash with its latest iPhone, the Android horde has been temporarily slowed, and Apple nearly caught up to the smartphone platform that is otherwise defeating it at every turn. Unfortunately for the iPhone, however, this turnaround seems to be temporary and there's already evidence that the iPhone 4S sales surge is over.

"The high-profile launch of Apple's iPhone 4S in the Fall had an enormous impact on the proportion of smartphone owners [in the United States] who chose an Apple iPhone," Nielsen reports. "Among recent acquirers, meaning those who said they got a new device within the past three months, 44.5 percent of those surveyed in December said they chose an iPhone, compared with just 25.1 percent [right before the iPhone 4S launch]. Furthermore, 57 percent of new iPhone owners surveyed in December said they got an iPhone 4S."

That's a huge jump for iPhone. And Android saw a corresponding fall in market share—which is to say a percentage of total units sold in a given time frame—from 61.6 percent of all smartphones before the iPhone 4S launched in October to just 46.9 percent in December. That was still enough for Android to edge out the iPhone, of course, but by a much lower margin than before the iPhone 4S launch.

(Other market researchers cast some doubt on Nielsen's numbers. A late November report from comScore claims that Android continued dominating the iPhone throughout last year, with 47 percent of US smartphone customers buying Android handsets that month, compared with just 29 percent for Apple.)

Regardless of the numbers, Apple is benefiting from a temporary bump due to a once-yearly major product launch. And there's already evidence that the sales boom is coming to a close. Wait times for the iPhone 4S were a week or more throughout the end of 2011, but with the New Year, those times have dropped dramatically on all three US-based carriers that sell the device, as well as on the unlocked version. In fact, many Apple retail stores now have the iPhone 4S in stock—no waiting required.

This is a problem because Apple isn't expected to debut its iPhone 5 until mid-year at the earliest. Meanwhile, the steady release of ever-better Android handsets on all carriers continues unabated. This is one of only many differences between the two products, and a factor that helps Android maintain and extend its overall lead against its slower-moving, single-supplier rival.

And other platforms aren't doing so well. Research In Motion's (RIM's) BlackBerry continued to hemorrhage users in the United States during the previous quarter, dropping from 7.7 percent of the US market in October to just 4.5 percent in December. Microsoft's Windows Phone remains mired at 1.4 percent.

Overall smartphone usage is only rising too, Nielsen notes. "As of Q4 2011, 46 percent of US mobile consumers had smartphones, and that figure is growing quickly," the company reports. "In fact, 60 percent of those who said they got a new device within the last three months chose a smartphone over a feature phone."