Whether you root for Google or Apple, it's a heck of a horse race as Android beat the iPhone in first quarter U.S. sales, according to the NPD Group.

Android sales accounted for 28 percent of smartphone sales last quarter, NPD reports. That puts Android ahead of the iPhone's 21 percent, and within striking distance of Research in Motion's BlackBerry, which took 36 percent.

It's worth noting that while Android had a great quarter, it still lags behind RIM, Apple and even Windows Mobile for total market share, according to recent statistics from ComScore. Google's operating system had 9 percent of the market as of February 2010, compared with the iPhone's 25.4 percent, so Android won't catch up for a while, if at all.

Still, as my colleague JR Raphael noted when ComScore reported its numbers, Android's growth is striking. Not only did Android outsell the iPhone, but it's the only smartphone OS whose unit share grew since the previous quarter. The iPhone, meanwhile, is flat, while Windows Mobile, BlackBerry and WebOS quarterly sales share is in decline. If this trend continues, Android will catch up to its competitors for total market share in a hurry.

But hold on, Android fanboys, before you claim a victory in the name of open source and open systems. Understand that Android's growth probably has more to do with aggressive sales and marketing by Verizon Wireless. The carrier expanded its two-for-one promotion last quarter to include all smartphones, and increasingly, that means Android phones.

NPD analyst Ross Rubin said in statement that this kind of carrier promotion is always a crucial factor in smartphone sales. Add the availability of Android phones on T-Mobile, Sprint and AT&T, and you've got an easy recipe for more sales than a single phone (iPhone) on a single carrier (AT&T).

With Verizon's new Droid Incredible selling out in its debut week, and Sprint's upcoming HTC EVO 4G gathering buzz as the next phone to beat, Android still has plenty of momentum. NPD's findings are just a milestone along Android's ascent.