Rumors
Sheesh! It's a never ending theme. I can see Apple coming up with some small, less expensive device to run iOS apps that seems more like a notebook that would use ARM chips. But does Apple want a device like that? They don't seem to be going in that direction.
But to supplant x86 for OS X seems like a stretch. No matter how powerful ARM can get, x86 will always be a magnitude more powerful. Will Apple be willing to sacrifice that power? I don't think so. Apple depends on powerful chips to run its graphical interfaces.
Now that the iPhone 5 is as, or more powerful than, the most powerful G4 PowerBook, some might say that it's powerful enough already for such work. But they would be wrong. Five years from now, an Apple Arm chip might be as powerful as an Intel i5 mobile chip today, but we will be well past that by then.
What I can see happening, is Apple emulating iOS, or having their API's in OS X. Possibly, even an ARM chip in the machine as well, as they are inexpensive, small and use little power.
But I can't see it taking over in OS X unless Apple is willing to abandon all computing tasks that require some real power.