NetMarketShare’s desktop Operating System Market Share metrics for December 2011 show the Mac OS slipping back another tenth of a point on the month, with Windows posting exactly the same share for two consecutive months. Linux made another proportionally significant gain, continuing its recent positive streak.

NetMarketShare has posted its Operating System Total Market Share metrics for the significant players for January, 2011 (December ’10 figures in parentheses) tracking changes over the the past month. Perhaps the most significant metric this month, at least psychologically, is that Windows has dropped below 90 percent share for the first time in a very long time. The Mac OS gained just short of a quarter of a point over the month, while the iOS’s share increased significantly for the second month in a row.

Steve Ballmer, already famous for his dancing skills, has pulled off a spectacular 180º pirouette in his position on Apple. In an email earlier this summer, Ballmer addressed Apple’s rising threat to Microsoft’s operating system dominance, telling staffers Apple’s success was:

Because they [Apple] are good at providing an experience that is narrow but complete, while our commitment to choice often comes with some compromises to the end-to-end experience. Today, we’re changing the way we work with hardware vendors to ensure that we can provide complete experiences with absolutely no compromises. We’ll do the same with phones—providing choice as we work to create great end-to-end experiences.

In the interview with Ann Winblad, a partner at Hummer Winblad Venture Partners, Ballmer neatly about-faced this position. Discussing current market-leader Nokia’s 30% position, Ballmer stated, “If you want to reach more than that, you have to separate the hardware and software in the platform.” This, obviously, is the same model that propelled Microsoft to dominance in desktop computing with the Windows OS, and it is exactly what Ballmer vowed to Think Different about in his memo!

Despite being relatively new to the competition, the iPhone is already a huge threat to the popular smart phone OS, Symbian. Despite the fact that Symbian has been around for about seven years, as opposed to the iPhone 3G’s two months on the market, iPhones may outnumber Symbian-powered phones in as little as two years.

Projections after the break.