There's been no "official" memory map released (I don't know if Notch has even thought that out yet), but people on the forums are working on that. There will be various peripheral controllers, add-ons, etc., and as time goes on a "standard deployment" of the computer system (not just the CPU) will likely fall into place.
tl;dr: It doesn't break compatibility 'cos there's no compatibility to be broken yet ^^
tl;dr: It doesn't break compatibility 'cos there's no compatibility to be broken yet ^^