Apple’s U.S. smartphone installed base has surpassed Microsoft’s for the first time

Click to enlarge. Source: comScore

Given that the iPhone has been outselling Windows Mobile devices in the U.S. for nearly two years, it comes as something of a surprise that Apple AAPL has only now caught up to Microsoft MSFT in terms of active smartphone users.

But that’s what the latest data from comScore show. The bar graph at right, drawn from numbers obtained by FierceDeveloper, shows that the installed base of iPhone users in the U.S. overtook Windows Mobile’s sometime between July and October 2009.

Microsoft had a big head start, of course, but its market share has been stagnating lately. Research in Motion RIMM , by contrast, enjoys both a head start and a growing market share. According to comScore, about 40% of the smartphones still in use in America are BlackBerries, while less than 25% are iPhones.

A copy of comScore’s report can be purchased at the compay’s site for $4,995.

Below the fold: A full-size version of the chart.

[Follow Philip Elmer-DeWitt on Twitter @philiped]

Source: comScore