During the rapid ascent of Google (GOOG) Android phones in 2010 it was never clear how much their popularity was due to the fact that Apple’s (AAPL) iPhone was available in the U.S. on only one carrier — AT&T (T) — with somewhat spotty coverage.

The test was supposed to come in February, when Apple finally released an iPhone for Verizon (VZ).

Four months later, the results are in. Although Android is still the most popular choice among recent smartphone purchasers, a Nielsen survey released Thursday clearly shows a leveling off of Android’s growth rate and a corresponding acceleration for the iPhone.

“Apple,” according to Nielsen, “is now driving smartphone growth.”

It’s not at all certain how much of this is due to the Verizon iPhone and how much to Apple’s increasingly popular iOS ecosystem, which got a boost from the launch of the iPad 2 in March.

But the damage for Android’s competitors is done. In a smartphone market that is rapidly expanding, Android had a 38% share of the U.S. market in Nielsen’s March-to-May survey, up two percentage points from February-April. Apple’s share was up one point to 27%, and Research in Motion’s (RIMM) was down two points to 21%.

According to Nielsen, 38% of Americans now own smartphones, and 55% of those who purchased a new handset in the past three months bought a smartphone (rather than a dumb one), up from 34% a year ago. See pie charts below.