Really aren't using the 'Mac' platform. They are using the hardware (PPC), and it just happens that Apple is the largest distrubutor of hardware that uses PPC CPUs.
People dont realize that MS has had Windows running on PowerPC chipset for years. They used to sell Windows NT for PowerPC and others. This is because NT (Windows 2000, Windows XP, XBOX) kernel runs on top of a Hardware Abstract Layer.
Obviously I mean natively so you don't have to have Mac OS. (Wouldn't that be funny? You having a Mac without OSX and with Windows? I think so!) And even then, Virtual PC doesn't run Longhorn. I don't care about XP I'm talking next gen.