Samsung announced that they sold 5 million Galaxy S II phones in just 85 days (on July 22nd). That's a lot faster than the Galaxy S which took 125 days to sell 5 million. The Galaxy S took 7 months to reach 10 million units and Samsung says that the S II is expected to reach that amount sooner too. And it's not even on US soil yet.
The SII sold 3 million units in 55 days, they sold 2 million in just 30 days, so sales are actually accelerating as Samsung introduces the phone into more and more markets. In this rate the will cross the 10 million mark soon, beating Samsung's own original expectations.
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Source: Galaxy Forums via Samsung Tomorrow