Windows Mobile, Microsoft’s increasingly embarrassing single entrant in the smartphone OS field, can’t seem to hold onto the attention of the world’s consumers. A new survey by market research firm comScore sees Windows Mobile dropping one place in the overall rankings, and Apple’s iPhone gaining one, which puts it ahead of Microsoft.

The iPhone climbs to No. 2 in terms of general smartphone OS market share in the U.S., while Windows Mobile drops to third. Research In Motion’s BlackBerry OS still tops the list, and in fact does so with an impressive gain in customers over the course of 2009, according to a breakdown of the report by DailyTech.

comScore compiles its results for the smartphone survey every three months, collecting data from thousands of U.S. consumers. The latest three-month period, the one which is detailed in this latest report, ended in October of this year.

RIM, as mentioned, saw pretty significant growth throughout the year. At the beginning of 2009, it had just under 10 million customers, and according to the comScore report, it now boasts 14.96 million in the U.S. RIM has been employing aggressive pricing strategies to compete with the iPhone’s success, including handset giveaways and deep discounts, and it looks like the BlackBerry maker’s efforts have paid off nicely.

Windows Mobile actually showed negative growth — it’s the only smartphone company that can make this unfortunate claim, and now has around 7.13 million users. Buyers could be shy of committing to a device that runs Windows Mobile 6.5 when version 7 is set to launch sometime early in 2010. Everyone else, including Palm’s WebOS, Google’s Android, and Symbian all experienced modest gains. Android still tails all others, but during the most recent period it broke the 1 million mark, and I’d expect to see it surge even further on the strength of the Droid in upcoming surveys.

As for the iPhone, it grew to 8.97 million users over the period ending in October. That’s a pretty impressive lead over Windows Mobile, though it means Apple still has a lot of ground to make up before it can snatch the crown from powerhouse RIM. It’s even more impressive when you consider that at the beginning of the year, the iPhone only had around 5 million users, meaning it experienced about 70 percent growth over the course of 2009, a performance that likely has a lot to do with the release of the 3GS and the significant price drop and continued sales of the 3G model.

Overall, the smartphone market continues to do well. Roughly 11.8 percent of the American population claims to now own one, according to this most recent poll.