The Mac will still be a trusted platform. i.e. even though future Macs will run on intel, the internals of a Mac will still be locked down by Apple, and most likely that is all they will support. I severely doubt that Apple will ever sell them with Windows, though in theory there's no reason why Windows wouldn't run on them. You might be able to get OS-X running on a PC, but prepare yourself for some serious hacking around - and Apple is never going to support it. Though, to be honest, since OS-X is built on top of a POSIX kernel (some flavour of BSD, I understand) this has probably been possible for a while.