The Nielsen data covers a period that includes a full-month of iPhone 4 availability. (The device became available at the end of June 2010).
Among all smartphone owners, Blackberry still holds the dominant share with 31 percent of the market, though its lead over Apple is declining, Nielsen said. Twenty-eight percent of smartphone owners have Apple iPhones, compared to 19 percent who have Android devices.
Technology research firm Gartner said last month that Android will become the number two mobile operating system worldwide this year, leapfrogging BlackBerry and challenging market leader Nokia's Symbian by 2014.
Gartner said Symbian and Android will account for 59.8 percent of mobile operating system sales by 2014.