[freehunter]

This could get into deep discussions about what is an OS (versus a kernel), what constitutes the "same" OS (versus a modified version of the OS) and even what can be put in the same class as Windows.

I think what Microsoft is getting at is, OSX and Ubuntu don't run the same code on their mobile devices. Windows 8 will. It's marketing, and NetBSD doesn't even cross their mind.

[rbanffy]

It all depends on where is the line defining what's an OS. A Linux machine doesn't need X to be a Linux machine - Unix machines have been serving terminals for decades. So much, in fact, I joke that, in order to be a serious computer, one has to have no monitor, keyboard and mouse ports - if you really need a physical console, a serial port will do. So, I've seen Linux running programs on a very broad selection of hardware, from ARM to zSeries (68020+ has always been more of a curiosity, albeit there were serious Unix machines using them).

Really, it's marketing. Not truth.