I think some kind of hardware emulation is a given but it will probably be less noticeable than Rosetta was (hardware is a lot faster and the lessons from Transmeta have been learned).
The other point: code that requires specific hardware acceleration will be a deal-breaker. To be fair Apple already has extensive APIs for this so it should be able to handle most of this at compile time. But I wouldn't make any plans about your future OS yet. Apple generally provides OS support for Macs for at least five years. This applies, of course, to all new devices sold from now until the switch which should give both them and developers time to work out any kinks.
Biting the hand that feeds IT
