Now, with Macs moving to Intel chips, it's a whole new ballgame. The new Macs will run Windows quite nicely. Last time I looked you could switch operating systems only at boot-up, but I suspect that's changed now, or will soon. A couple of years from now I would be very surprised if advanced Macs couldn't easily run both OSX and a Windows windows at the same time, and not just for high-powered geeks.

As for OSX (which is pretty much Unix in a Mac Suit) running on a PC, I think the issue now would be more about peripherals and drivers rather than the main operating system (might be true for Windows on a Mac, too.)