Agreed Apple has a lot of experience in this area. Under the hood every iOS app is a fat binary, since ARM isn't just one architecture. Nor are the variants compatible with each other like x86/PowerPC. I just don't see evidence ARM is powerful enough currently to make the jump and allow a smooth transition (At Apple's quality/polish level). Nor can I see Apple being willing to drag this out across a multi year timeframe.

For PowerPC to x86, they had a clear transition path for every product. Low end went to Core processors. High end went to Xeon. They also were able to license the binary translation layer Rosetta (it wasn't an in house technology).