just need to match perf on laptops
the ARM solutions only have to match the performance of the Intel based laptops, not all of Intel's CPUs. With ARM chips already hitting 1.5GHz the current die shrink methods show it'll hit 2GHz in no time. Pair that with how small and power efficient multi-core ARM chips are as opposed to the best Intel can do you should see why Apple might go this route.
And if I were Apple and have seen demo's of what these multi-core high speed chips can do and had a tablet hardware they wanted to merge with their desktop over a few years then this is a no-brainer. ARM is already scaling up and it's pretty obvious that the future of CPU design is the multi-core method it makes sense and cents. I'm not much of a fan of Apple but they don't seem to be on the wrong path for profits and efficiency very often. Add their App Store for desktop and laptops and you also have a way to get ARM or x86 based apps to customers as needed without the customer knowing which one they need since the App Store infrastructure will figure that out and deliver the correct version for your hardware.
I also recall recent news of Apple teaming with Intel to build their ARM chips instead of Samsung doing it. If that means using Intel's new processes( 32nm, 22nm or even 3D 22nm ) then there's lots of things the faster smaller ARM chips will do.
It would be great to finally start to see ARM laptops as long as their boot systems are not locked to the OS. I also wonder how much Google ChromeOS has to do with all this since it could be what's triggered Microsoft Windows for ARM and that's triggered all the new interest in ARM in PC sized devices.