[...] I would think we would need to see decent 64bit ARM processors before Apple could switch.
The big difference I see between the PowerPC to Intel transition and a possible Intel to ARM transition? The transition won't be as rushed and there would probably be current Intel and ARM versions of some systems.
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Agree with all your points. I tend to gloss over the 32-bit / 64-bit "quantum leap." But ARM announced their ARMv8 64-bit processor architecture last October, with more specific details to be released this year. They suggest that consumer devices using 64-bit ARM chips might become available in 2014.
And it's not just Apple that's thinking about switching. Microsoft seems committed to ARM-based tablets to run a variant of Windows 8. And that, too will require 64-bit SoCs.
As for the co-existence of Intel- and ARM-based Macs, yes, I think the transition needs to be handled very carefully. (But Apple has handled do-or-die transitions successfully in the past: 68k->PowerPC, OS9->OSX, PowerPC->Intel.) So maybe the MacBook Air line could use 64-bit ARM chips exclusively, and the MacBook Pro line could use Intel chips exclusively. The MacBook Air would become mechanically simpler, with no fans and less need for cooling of any kind. Mechanical simplicity translates to lower component and assembly costs, which together with the lower cost of the ARM chips would help keep the MBA's margins up while allowing a lower average selling price. All bad news for the Ultrabook wannabes of the world. All great news for highly mobile pros and consumers.
Meanwhile, the MacBook Pro could once again be positioned for the "pro" market. The unfortunate minority who are still forced to use legacy Intel-based productivity suites like the ones Adobe and Microsoft still ship. Because they're paid to do it. Some of them might even run Windows on their MBPs. Again, only because it's needed for work.
Going a little further down the road, Apple could eventually migrate the iMac line to ARM chips. That would give Apple even better economy of scale for their ARM-based Ax chips, and again, most consumers wouldn't need to run Windows or heavy productivity suites on their iMacs. It would crystallize the difference between the consumer and pro lines of MacBooks and Macs.