Well, Apple certainly knows all about fat binaries and supporting one architecture on another (Rosetta), and their toolchain and OS kernel certainly support ARM today. It's just a matter of whether their in-house ARM chips offer a sufficient benefit over Intel's chips that it would be worth the pain. On the one hand, you'd have to support legacy x86 software and deal with that transition; on the other hand, you could run iOS apps natively, and you might be able to eek out longer battery life.

Amusement: Google has a much easier time of this, with running Android apps on ChromeOS devices (no, really), since Android apps are distributed as Dalvik bytecode which is then compiled at install time for the local machine architecture. Thus, most Android apps don't know or care whether they're on x86 or ARM.