Pure speculation on my part, but I think Apple wouldn't want people installing either other OSes on their machines or their OS on other hardware, so I would expect some lock-in hardware/firmware/software to make impossible or really difficult to build ARM hackintoshes and the other way around. Hoping to be wrong, though.
In his interview with Gruber, Federighi explicitly said that they wanted hobbyists and hackers to be able to play with the Mac hardware, so it would be possible to boot other OSen. Don’t have a link right now, but it was clear that he was saying that the platform would be hackable.
