Microsoft Corp (MSFT)

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New market research from The NDP Group confirms Android phones outsold the iPhone last quarter in the United States. RIM’s Blackberry OS remained at the top with 36%, while Android jumped to number two with 28%, followed by the iPhone OS at only 21%.

Now that all four major U.S. carriers (and several regional) are selling Android phones, we can’t say this comes as a surprise. A dozen smartphones across all the carriers should result in higher sales (just look at Blackberry).

The real question now, is how long will the iPhone remain an AT&T exclusive? Engadget is reporting that Apple and AT&T signed a five-year deal that reportedly does not end till May 21st, 2012. Can Apple afford to keep the iPhone locked to a single U.S. carrier while Android adoption continues to rise?