NPD's numbers come from more than 150,000 self-reported online consumer surveys, so there's bound to be some variance between their results and actual sales data. But the numbers are still surprising considering comScore showed Android with a 10% share in February. NPD is usually considered a reasonably accurate source, so if their figures pan out among other research firms such as IDC and Gartner it'll be a stunning feat for Google's platform.
It's important to note that there are numerous Android-based smartphones on the U.S. and a handful of carriers offering them. Apple's smartphone, on the other hand, is only currently available from AT&T as the 8GB iPhone 3G along with the latest generation 16GB and 32GB iPhone 3GS. Much of Android's gain is attributed to Verizon, whose success with the Droid line and Buy One Get One promos helped keep its smartphone sales very close to AT&T's.