Well, a significant part of that work has already been done in producing iOS (no, its not OS X but you can be sure that it shares as much code as is feasible). And most applications (including big chunks of the system) written in C or Objective C would just need re-compiling. Its a far cry from the days when everything had to be written in lovingly hand-crafted assembler. Many of the Linux distros, such as Debian, manage to support ARM, x86 and PPC.
I'd be surprised if there isn't already an ARM rig running OSX somewhere at Apple, just as they had x86 OSX running long before the switch from PPC (how far it is from being production-ready is another matter). Its a good insurance policy against ARM beating Intel in the ultra-low-power mobile stakes.
...however, the iMac is the last place the ARM is likely to turn up. The ARM's 'Unique Selling Point' is low-power, so if Apple went for OSX-on-ARM it would be for something that sat between the iPad and MacBook Air in the product range, or a low-power home server.
I'm saying it's likely - but it is feasible.
However, my best guess (pure speculation) would be a hybrid ARM/Intel subnotebook, with the ARM providing 'always on' communications (email, facetime, messaging, note-taking, music, book reading etc.) and the Intel CPU firing up to run 'serious' OS X applications. Think of a MacBook Air with the top cover replaced by an iPad and elaborate from there...