The rumor that Apple is considering the use of ARM-based processors in future generations of Mac OS X systems is an interesting one, but I'm just not sure how practical such a move would be.
If the rumor proves to be true, that would be Apple's fourth processor change since the introduction of the Macintosh in 1984. The move from the Motorola 68K family to PowerPC in the mid 1990s was a rather bumpy ride, because - to use one of my favorite automotive metaphors - Apple was trying to pull off an engine change without slowing down, to say nothing of pulling off the road.
When Apple switched from PowerPC to Intel CPUs in 2006, it was a much more graceful transition, if only because the core of Mac OS X had originally been developed for Intel x86 chips in the first place. As far as I'm concerned, the only mistake Apple made in that switch came from using Core Duo processors at the heart of those first Intel-based Macs. When the company released the Snow Leopard version of Mac OS X in 2009 with the promise of 64-bit support, the Core Duos were limited to 32-bit execution. (So much for the argument that Apple's machines held their value longer than did computers from Dell, HP and other "Wintel" vendors.)
Now, it's whispered that Apple sees an opportunity to move some of its hardware, especially lower-horsepower devices such as the MacBook Air, to ARM. But I'm not sure what that switch would achieve, beyond annoying Intel.
Although swapping Intel CPUs for ARM processors would have the benefit of aligning the ultra-portable Air more closely with the iPhone and iPad, I just don't see it being a smart call for Apple. That's because the low-powered CPUs used in the Air would have to support - at least for the first few years -an additional abstraction layer in order to run the same software that exists for the MacBook, MacBook Pro and Mac Pro.
Apple relies heavily on customer goodwill to offset the premium prices it charges for its desktop and notebook computers. It was hard enough to justify Snow Leopard's inability to work on a PowerPC-based Mac, or to tell end users to buy a new printer with Snow Leopard, because the printer manufacturer didn't want to go to the trouble of writing new drivers for that OS. It's another thing to support separate code bases for an OS depending on the processor. Apple should have an idea of how much work that is after its experience during the early years of its move to Intel hardware, when it had to do just that for the last half of the Tiger series and the entirety of the Leopard releases.
I could see an ARM processor going into a successor to the MacBook Air, thus turning it into a super-iPad. If the forthcoming Lion release of Mac OS X - which is going to feature an iOS-like look and feel - proves to be a success with users, then it might make sense for Apple to consider an iOS device as an Air replacement. But until we see how customers react to Lion, and the interface changes therein, I wouldn't be putting too much stock in this rumor.