This all sounds like one of those brilliantly original ideas, like Esperanto or Metric Time, which, for all their ingenuity, come up face-first against the brick wall of pragmatism and inertia, people being creatures of habit and all; except for two things: firstly, in the age of GPS, Natural Area Coding System codes will be easier to determine than traditional post codes, as there will be no maps or databases to look up, and also no questions of changing boundaries (as happens from time to time, for example when resident groups or landlords lobby to redraw the boundaries so their properties aren't in Footscray or South-Central LA or somewhere similarly insalubrious). And secondly, Microsoft are backing the idea, and plan to integrate it with their MapPoint map service. (Perhaps they hope that they can use it to lock customers into depending on Microsoft technologies or something? I wonder if it's patented.)
NACS could eventually become a world-wide postcode, replacing legacy postcode systems devised in the pre-GPS era. It would probably happen with shipping/courier firms utilising the codes internally, and businesses which deal with them putting them on their details, and finally postal services phase out the pre-GPS system and adopt this. Of course, if someone detonates an EMP bomb in the high stratosphere and fries all the GPS satellites (or if the Americans stop being our friends), we're all fucked; but if all the satellites disappear, we'd have more important things to worry about.