Since it's currently fashionable to explain that one switched when Apple uses TPA - or whatever it might be called in the future: first wait and see. See what Apple does and how - there are always rumors beforehand.
If TPA is actually included: Linux can also be a usable system, even if the interfaces are quite sick (although current XFCE versions don't look that bad) and if there is no more PPC in Apple hardware and you put Linux on it: you can also buy your notebook from IBM. They have nice devices that also work very well under Linux.
And last but not least: just because new Apple hardware is different, it doesn't change the already purchased hardware - and Apple-typically, this usually lasts a few years longer. And under Linux, some Macs even run faster than under OS X.