Despite its rather steep $899 entry price – and Microsoft's inability to keep the tablet in stock – the company has moved more than 400,000 Surface Pro tablets since its February 9 debut. This information comes from anonymous sources in contact with Bloomberg.

While Microsoft is no stranger to making peripheral hardware and even game consoles, Surface represents the software maker's first DIY foray into the cut-throat business of producing an computers. With PC sales declining for the first time in nearly a decade, the stakes may be high for a company who depends on PC sales to move its products.

The relatively swift uptake of Surface Pro tablets in comparison to its much less expensive RT sibling is somewhat unexpected. However, the Surface Pro (and Windows 8) did bring something novel to the tablet space though: a desktop experience.

All told, Microsoft's Surface RT and Surface Pro sales add up to roughly 1.5 million units since October 26, 2012. Although 1.5 million is impressive by most everyday measures, it pales in comparison to competitors like Apple and Google who move millions upon millions of iPad and Nexus 7 tablets each month. In fact, Apple shipped nearly 23 million iPads in just Q3 2012.

In 2012, Microsoft projected it would move about two million Surface RT tablets in December alone. In hindsight though, that's nearly twice as many Surface RTs Microsoft has sold over the course of nearly five months.

Despite Surface's less than impressive sales, it seems Microsoft remains fully committed to the product. In November, Microsoft doubled-down on Surface, guaranteeing four and a half years of support. Recently, Microsoft CEO Steve Ballmer admitted to Surface's relatively sluggish sales but said its tablets are real business – even if they don't dominate the sales charts.