(We already know Apple is more than capable of executing CPU transitions on the Mac. I suppose the only other question is how much people value Parallels and Boot Camp.)
Here's what I think will happen - the Macbook Pro line will be slimmed down to Air specs, the Air that exists today gets discontinued to make way for an all-ARM version.
The Air brand will be the ARM laptop line, the Macbook Pro's will be x86 for a few years before they too switch.
Wow, I hope not. Right now I use MBP as a desktop replacement. I've been able to get away with that so far because the iMac isn't normally dramatically far ahead of the 17".
And unlike the transition from Power chips to x86, ARM won't be so much faster that something like Rosetta will be a workable transition solution for real applications -- effectively everything will have to be ported, or it won't work.
The epic volumes of ARM chips make it possible to overcome the manufacturing and scale issues, but doing complex super-custom ARM chips for PCs drops the chips back out of the mass market again unless an enormous number of PCs get converted all at once -- though this is something Apple is capable of.
Overall, there are enough logistic issues involved that I'm not convinced that ARM is the future of PCs unless the current ARM devices like iPads and Android tablets grow up to be our PCs of the future.