The Android mobile operating system is now the second biggest in the US, by sales

"The Android operating system (OS) continued to shake up the U.S. mobile phone market in the first quarter (Q1) of 2010, moving past Apple to take the number-two position among smartphone operating systems, according to The NPD Group, a leading market research company. NPD’s wireless market research reveals that based on unit sales to consumers last quarter the Android operating system moved into second position at 28 percent behind RIM’s OS (36 percent) and ahead of Apple’s OS (21 percent).
“As in the past, carrier distribution and promotion have played a crucial role in determining smartphone market share,” said Ross Rubin, executive director of industry analysis for NPD. “In order to compete with the iPhone, Verizon Wireless has expanded its buy-one-get-one offer beyond RIM devices to now include all of their smartphones.”
Strong sales of the Droid, Droid Eris, and Blackberry Curve via these promotions helped keep Verizon Wireless’s smartphone sales on par with AT&T in Q1. According to NPD’s Mobile Phone Track, smartphone sales at AT&T comprised nearly a third of the entire smartphone market (32 percent), followed by Verizon Wireless (30 percent), T-Mobile (17 percent) and Sprint (15 percent)."
Source: Press release from NPD Group, 10th May, 2010