Just a minor point, you can upgrade the memory on Macs.
I think the iPad has its place, but the number of people that can simply dump their computers for a "post-PC" device seem small, most people seem to be using them in conjunction with PCs, not replacing PCs altogether.
Apple does seem to have a plan to improve on the platform's limitations. I don't think it's a post-PC world yet. It looks like there are about a billion computers in use right now, 90 million iPhone OS devices doesn't quite compare yet. Not only that, the iPad requires a cable hook-up to a computer to activate it, which seems pretty unnecessary and I think shows that the platform isn't really post-PC if it's still stuck with a PC umbilical cord.