Apple: Intel Inside?

It's true, Steve Jobs just announced that Macs will now shift from IBM PowerPC processors to Intel. Before, it was the PowerPC processors that set Macs apart from the Windows PC (aside from the wonderful design of course), but now, it has been revealed that the Mac OS has been running on Intel processors for some time now in the Apple labs. I wonder what will prevent Windows users from booting up their PCs with Mac OS now that they are using the same processors?