Apple left PPC because PPC wasn't interested in Apple. For years Apple tried to convince the world that G5's were somehow mystically magically better than the equivalent X86 architecture running at faster speeds. It never really worked. And of course IBM abandoned Apple, they refused to produce a G5 mobile on any kind of reasonable timetable.
So Apple moved, any the myth of G5 was broken when even Rosetta applications ran faster on X86 than a G5.
I suspect the talk of ARM is based on the success of iOS devices. OS X and iOS are pretty much two heads of one beast. Under the bonnet there's no real difference. It makes sense to at least consider bringing them to the same architecture. You can get rid of some of the doubling that way. And with Microsoft producing an ARM version of Windows, you can still dual boot/run a virtual MS Windows session on Apple equipment. I wouldn't be surprised to see MS Windows ARM desktops in the future.
Apple always plays 3 to 5 years out, it's why they are still here and Amiga is coming back as a boring Chinese Windows box. You don't sit on your laurels. Todays hot is tomorrow's retro nostalgia. I can't say that they will make the move to ARM, but I can say they'd be stupid not to at least put it on the table for discussion.
