Doesn't matter. ARM based means you are stuck with whatever ARM provides. Whether it's Cortex-A9 or Cortex-A15 doesn't really matter.
All ARM provides is
designs for everything from complete chips to "mix'n'match" building-blocks to allow fabricators to assemble their own custom chips. It's also quite feasible that ARM would work with a big customer like Apple on developing new designs - at one point Apple owned a substantial chunk of ARM, and the chips used in the Newton were an Apple/ARM collaboration.
Plus you need all the software to support ARM as well. That's too much for just one computer, Apple would go ARM-only if they did that.
This is far less of a big deal now that most software is written in high level languages, if not scripting languages, and everything works through hardware abstraction layers. For an awful lot of software, supporting ARM is, in theory, just a matter of changing a flag and re-compiling - or compiling to a virtual machine-based byte code (which AFAIK xcode already offers - as does Microsoft .Net). Note that Linux and many of the big open source projects which form the Linux ecosystem happily support x86, PPC, ARM and other processors.
Of course, reality is never quite that simple, but I'm willing to bet that following Apple's development guidelines will go a long way towards ensuring this works. Apple could probably have the entire OS X App Store ARM friendly in record time. Its things like MS Office and Adobe CS (not to mention all those Photoshop plug-ins) that would take time.
I'd actually be very surprised if, behind some locked door on Infinite Loop, they don't have an ARM-based OS X system lashed together. That doesn't mean it will ever see the light of day.
If Apple identify a worthwhile share of the market that would happily restrict themselves to the contents of the OSX App Store in exchange for longer battery life/reduced weight - and maybe iOS compatibility thrown in - then an ARM-based Air might fly. Feasible, but maybe not likely.