iPhone has been the second best selling smartphone in US behind all the RIM handset combined for past two years. But now, according to the latest data from ComScore, iPhone OS’s active user base in the United States in October has surpassed Windows Mobile OS user base to capture second spot behind RIM in Smartphone OS market share in US.

RIM came in first with 14.9 million active users, followed by Apple with 8.9 million. Windows Mobile was in third place with 7.1 million, Palm was in fourth place with 2.8 million, Symbian was in fifth place with 1.2 million and Google Android was in sixth place with 1.02 million active users. All Smartphone OSes have seen growth in 2009, but iPhone’s growth has been exponential since the launch of iPhone 3GS in June 2009.

This is really bad news for Microsoft as US was their biggest market for Windows Mobile OS. And also a smartphone OS that was launch just two years back has surpassed the user base that Microsoft created in almost 10 years. Sales of devices based on Windows Mobile OS are declining, whereas, according to latest estimates, Apple will sell around 10 million iPhone units this quarter, a jump of 35 percent over last quarter.

Another bad news for Microsoft is that many analysts consider the latest Windows Mobile 6.5 release from Microsoft as second grade citizen of smartphone OS world. It looks like in 2010 the smartphone war in US will be between RIM and Apple with Microsoft, Palm, Symbian and Google watching from the sidelines.