i dont see why this debate has to become all "legal or not legal", its more on morals and the mindset of the person who wants to do it. if they want to change the CPU then they have clearly thought about the implications of their actions and accepted them. if they assume that they are covered by warranty and they are indeed not covered by warranty then they have only themselves to blame.
personally i think i would try it, especially if future CPU upgrades allow for it (which it looks like they will) and apple supports the CPU. i would probably wait for the warranty to be void first though
moviecutter: what would happen if say, i upgraded the CPU, then something went wrong (e.g. screen dies, or something unrelated) and i changed the old CPU back. there was no evidence of swapping the CPU. how would that go? is it easy to tell if somebody has modified the computer?