Josh Centers, writing for TidBITS:
Pundits have long speculated that Apple is working on ARM-based builds of Mac OS X that would allow the company to use its proprietary A7 CPU (or successors) in Macs as well as iOS devices. This is nothing new for Apple, which secretly compiled Mac OS X for Intel-based hardware for years while Macs shipped with PowerPC CPUs. But we hadn’t realized how far this work had come until a source inside Apple tipped us off to the fact that Apple is testing OS X 10.9 Mavericks on current iPad hardware. Not only that, but the ARM code is embedded in the shipping version of Mavericks. After months of attempts, we’ve cracked how to install and run Mavericks on the iPad Air. As Josh mentions, this is a total hack. I'm not using my Air as a guinea pig.