Reports in Bloomberg have suggested that we'll see Apple replace the processors in their Mac range, from the current Intel range to a ARM solution.
Back in 2005 Apple moved from propriety processors to Intel ones in their Mac range - this is why you can 'Hackintosh' most Intel based computers now - they use the same parts as everyone else.
Of course Mac OS X wouldn't just start working on ARM chips natively - Apple will have to re-code huge parts of the operating system to allow it to utilize them. However if they did, it could clamp down on the 'Hackintosh' market and would allow the company to invest in research and development for both mobile and computer devices. It would also allow iOS to run on Macs and OS X on iOS devices. OS X isn't nearly as popular as iOS, so there aren't as many applications for it - if the two were merged, OS X users would have access to a huge library of applications.
The benefits of ARM chips is their low battery usage - they aren't the fastest of computing chips but that isn't needed in the ultra-portable market like the MacBook Air. Take a look at the iPad - it gets 10 hours battery life, MacBook Air? 4-5 hours. That's in part due to the ARM vs Intel chip.