All Things D has this story on the possibility of Apple switching to ARM processors for their Mac computers. Arik Hesseldahl does a great job of laying out the pro, cons and possibilities.
In my mind, the compatibility with Windows would be the biggest reason there would not be a switch. Apple still makes the best Windows laptop in my opinion. :) But if you think about it, Apple could test the concept on a couple products. For instance, the Mac Mini would seem a likely candidate, especially a server build. A low power, multiprocessor machine would make a lot of sense. The other option would be with the MacBook Airs. The right kind of ARM build could give the machines some really amazing battery life.
I would not expect any wholesale switchover anytime soon. One point I think the story missed. Intel's mobile processors are going to get better. Intel is not going to stand still and left ARM take all their business. Expect the fight over low power and high performance chips to heat up in the coming years.