Apple and Microsoft are seen as long-time rivals of one another, but there are many programs and applications that can work on both a PC and a Mac. Programs like Microsoft Office, Adobe Photoshop, and many popular web browsers such as Google Chrome and Mozilla Firefox are just some of the examples of software that can work on both operating systems. However, a Microsoft Surface Pro user has gotten OS X (Mountain Lion) to run on his device, which is different because it’s an entire operating system as opposed to a single program.
Although PCs are seen as cheaper and more customizable, some users decide to use OS X as their operating system rather than Windows. There are some unofficial modified versions of OS X available out there, despite the Apple-imposed checks that determine whether or not the device is actually a Mac, requiring that the hardware is similar to hardware found on Mac computers.
The Surface Pro is powered with an Intel Core i5 and hardware that is similar to the hardware found on low-end Mac devices. Therefore, running OS X on this device could bypass the Apple-imposed checks. If OS X actually does run on this device, it would be left to see how much of the hardware would be supported. Two pieces of hardware, namely USB 3.0 and a multi-touch screen, are present on Windows devices, but are not present on Mac.
Source: Redmond Pie