I totally think Apple will go to ARM, but I can't see Apple doing shift until OS X is getting similar performance on ARM over x86. When Job introduced the x86 switch, he said they had had an internal version of OS X running x86 since day 1. I'm sure they have a version on ARM as well right now, and are just waiting for them to get "good enough". Apple's marketing on recent iOS devices has been also very interesting. Lots of use of the phrase "Desktop Class". Also, the GeekBench scores, and AnandTech teardowns support the hype: Apple's ARM chips truly are best-in-class and rapidly approaching mainstream x86 performance.
But the big difference between POWER->86 is that Apple now has majority of their products on a different architecture. Apple is a company that likes simplicity. They are going to want to have everything on one hardware architecture. As laptops get thinner they will basically be able to use identical hardware in iPads and laptops. It will just be different hardware exterior and OS.
And on the desktop and server side, even longer. I doubt we will ever see a 64-bit ARM part that turbos up to 4GHz+ with haswell/broadwell-like IPC. You can say it's because the desktop market is dead/declining/unnecessary now/etc, but it's also because nobody else is able & willing to match Intel's superb physical design. It's a shame that Intel management has failed to monetize that more effectively.
As demonstrated by things like iPad Pro, ARM is already more than capable for the things 90% of users are going to do. Realistically Apple would be best of with an intel based Mac Pro, but Apple likes to keep things simple, so I think they will just beef it up with loads of ARM chips clocked as high as possible with plenty of cooling or something.