Android continues to gather momentum in the U.S., retaining its title as the most popular smartphone operating system in the third quarter of 2011, a new study from A.C. Nielsen found.

Google's mobile operating system accounts for 43 percent of U.S. smartphones, up from 39 percent in July, while Apple's iOS has remained steady since then at 28 percent. Even though Apple's OS share didn't increase or decrease, the Cupertino, Calif.-based company was still the top hardware manufacturer in the quarter, with its iPhones used by  28 percent of all smartphone customers.

Rounding out the pack was Research in Motion, the maker of BlackBerry, with 18 percent market share, and Microsoft Windows Mobile, which only made up about 7 percent.  Both companies' figures were down from July, when RIM's OS accounted for 20 percent of the smartphone market and Windows 7 accounted for 9 percent. Microsoft hopes to turn it around next year when its partnership with Nokia finally bears fruit in the form of five new smartphones.

Nielsen also reported while 43 percent of U.S. mobile subscribers own a smartphone, the vast majority of smartphone users are under 44. Nielsen reports 62 percent of adults between 25 and 34 use smartphones, while the 18-24 and the 35-44 age brackets are similarly hover around 54 percent penetration.

While the third quarter results are interesting, the real test will come in the fourth quarter as the smartphone makers battle it out in the holiday sales. Apple will sell customers the iPhone 4S, while Google-based Android will be pushing several smartphones from Samsung, like the Galaxy S II, as well as Motorola's Droid Razr. Unfortunately for U.S. users hoping to snatch up a Nokia Lumia 800, Nokia's Windows 7-based phones will only be available in Europe in time for the holiday season. Microsoft and Nokia will release their smartphones in the U.S. in early 2012.