Android extends its lead as BlackBerry fades, WP7 stalls

Android (s goog), despite some mounting legal challenges in the marketplace, is flying higher than ever, according to Nielsen (s nlsn), which said Google’s operating system is on about four out of every 10 smartphones sold in the U.S. Nielsen’s second-quarter smartphone share numbers conclude that Android has moved up to 39 percent of the smartphone market, up from 29 percent between November and January, a rise that has come at the expense of BlackBerry (s rimm), which fell from 27 percent to 20 percent over the same period.

Apple (s aapl) continues to hold its ground at 28 percent of the smartphone market, and it leads by a healthy margin as the largest smartphone manufacturer. Apple’s OS market share has grown one percent since the November to January period. Meanwhile, Windows Phone 7 (s msft) and Windows Mobile lost a point over the same period, falling from 10 to 9 percent. The drop highlights Microsoft’s struggle to gain momentum for its new WP7 platform. It’s unable to offset losses of its waning Windows Mobile devices with Windows Phone 7 sales. That will have to change with the upcoming Mango software update and Nokia partnership if Microsoft wants to compete.

Looking at the manufacturer numbers, HTC trails Apple with 20 percent market share, with 14 percent made up of Android devices and 6 percent running Windows Phone 7 and Windows Mobile. HTC is tied with RIM, which has stumbled but has managed to hold on to a decent market share despite a paucity of new devices. Motorola (s mmi) is next in line with 11 percent, including 10 percent on Android. Samsung, at 7 percent (5 percent on Android and 2 percent on WP7) is the one to watch, because of its upcoming U.S. launch of the Galaxy S II, which has already sold 5 million units in just under three months.

It’s interesting to see the list of top Android manufacturers because it’s composed of exactly the companies Apple is pursuing legally for patent infringement. The leaderboard could change significantly if Apple succeeds with its claims. Analyst Toni Sacconaghi of Sanford Bernstein wrote in a note today that Apple is looking to “upend” Android’s momentum. And if it prevails and forces Android makers to undertake costly or difficult workarounds, it could help pump up Apple’s market share by up to 10 percentage points, Sacconaghi believes.

Overall, it looks like there’s still a lot of volatility in this market. IDC has put out some big projections that have Android and Windows Phone 7 on top by 2015. But we still have to see how the legal battles play out, what kind of real oomph the fast-fading Nokia (s nok) brings to Windows Phone 7, and what kind of turnaround BlackBerry can muster. While it seems like things are settling into a predictable rhythm, this story may have some twists and turns ahead.