The 32 to 64 bit issue really isn't one, its a CPU, make the code run on it. Same rules apply for both MS and Apple. As you mention, the main issue is in the drivers which come in all flavors in the Wild West style when the controllable space is unlimited vs one that has been tested and approved. ie. you don't have 100 companies out there making motherboards with various chip-sets which require special drivers etc.
I have spent many many hours in linux with different MBs trying to compile drivers to make machine X work. With Microsoft, its not so much compile drivers as it is try and find one someplace that works. On the Mac, I because of the way they do things, I have never had a driver issue. On all the built in hardware is a given, but even attached things, I have not had to think about I need to d/l some driver or something.
Now back to the subject line, its interesting to think about, but I just don't see an ARM macbook in the near future, there are many issues with 'why' and 'how'. it could happen at some point, but I don't see a target market for it right now.