According to a Thursday-released report by market research firm comScore, the Apple iPhone has, for the first time, surpassed Microsoft’s Windows Mobile in terms of US market share of smartphone operating systems, making the iPhone America’s ‘number two’ smartphone, behind Research In Motion (RIM)’s BlackBerry.

The comScore survey gauged the market share of the different operating systems in the country by questioning mobile subscribers, over 13 years of age, about the phone that they used. Thereafter, figuring out the operating system their devices ran on, comScore calculated the market share statistics for the recent report.

ComScore, which averages results every three months, found that Apple’s number had risen from 5.2 million in February to 5.7 million in May, 6 million in July, and 8.9 million in October. Meanwhile, the numbers for Microsoft’s Windows Mobile stood at 6.8 million in February, 7 million in May, 6.6 million in July, and 7.1 million in October.

Meanwhile, the market share of the RIM BlackBerry showed a steady ascent in its installed base - from 9.6 million in February, 12.2 million in May, 13 million in July, and 14.9 million in October.

The comScore statistics corroborate the trends being reported by mobile ad firm AdMob, which had earlier said: “Users of the Apple iPhone were most likely to consume mobile media, with 94 percent of users doing so in September 2009.”