I think this transition will be done for their consumer products only. Most consumers nowadays just buy MacBook Pros as their laptop even though they don't need the raw speed it offers. They don't really need those special intel instruction sets and they don't need CPU enhancements for multimedia encoding and decoding.
They want battery life. They want a snappy interface. They want to run Safari/Firefox/Chrome. They want to write in Word/Excel/PowerPoint. They want to use full fledged iLife.
And ARM + GPU + good hardware acceleration can offer this better than x86-64.
The AppStore creates a great way to cover ther architectural differences. Buy it once, and automatically buy it for both.
I am all for keeping the MacBook Pro a Pro-machine with Intel processors and all bells an whistles. But ARM could be big for the consumer space.
Bingo. An ARM-based MacBook Air is unlikely in 2012 but could debut in 2013. Boasting great all-round performance, compatibility with iOS and OSX apps, with unbelievable battery life. It could be a hybrid touch/keyboard device.
On the point of "CPU enhancements for multimedia encoding and decoding", there's a lot of that already in the iPhone and iPad. Hence the Garageband and iMovie apps for iOS. Not to mention all the hardware decoding of audio and video that iOS devices have been doing for years.