Gartner added to the heap of evidence that shows Android is the fastest growing mobile OS in the industry.

According to Gartner’s second-quarter numbers, Android gained 15.2 percent of total smartphone OS market share year-over-year. Gartner put Google’s Android OS at just 1.8 percent of total smartphone market share in the second quarter of 2009. That’s jumped to 17.2 percent in the second quarter of 2010.

Android overtook Apple’s iPhone OS to become the No. 3 OS in the world. In the U.S, it also overtook RIM’s OS to become the top smartphone OS in the region.

“A non-exclusive strategy that produces products selling across many communication service providers (CSPs), and the backing of so many device manufacturers, which are bringing more attractive devices to market at several different price points, were among the factors that yielded its growth this quarter,” said Carolina Milanesi, research vice president at Gartner, in a statement.

Overall, Gartner found that worldwide mobile device sales to end users totaled 325.6 million units in the second quarter of 2010, a 13.8 percent increase from the same period in 2009. Smartphone sales to end users accounted for 19 percent of worldwide mobile device sales, an increase of 50.5 percent from the second quarter of 2009.

Still, average selling prices were down significantly on increased competition in the market.

“Although the mobile communication devices market showed double-digit growth this quarter, average selling prices (ASPs) were lower than expected and margins fell,” Milanesi said.

“We attribute the decline in ASPs to a stronger dollar, a depreciating euro and intense competition that drove price adjustments and changes to the product mix.”

The second quarter also saw some movement in the Top 10 mobile device manufacturer rankings. HTC made its debut in the Top 10 worldwide ranking, holding the No. 8 position with 139.1 percent growth year-on-year. Gartner said the change reflects the popularity of its Android portfolio but also a more aggressive branding strategy compared to the same period in 2009.

“Crucially, as we predicted, the sudden growth in media tablets, such as the Apple iPad, did not appear to hold back smartphone sales. We believe that most tablet users still feel the need for a truly pocketable, yet highly capable, device for those situations when it’s inconvenient to carry a device with a larger form factor,” Milanesi said.