Steve Jobs says that a reason for the move is that Intel chips now offer much more power per watt than PowerPC chips. Which sounds strange, given that Intel chips are encrusted with legacy backward-compatibility baggage dating back to the IBM PC, whereas the PowerPC platform is free of such constraints. Though perhaps, because of Intel's far greater market share, enough was invested in getting around this.
Jobs also revealed that MacOS X had been built internally for Intel platforms for the past five years (which is plausible, given that NeXTSTEP, on which it is based, was sold for Intel machines). Which goes some way towards explaining the somewhat backward choice of architectures: i.e., it's inertia.
Of course, this does not make it any less of a missed opportunity to provide a computing platform on a new, unencumbered architecture. Especially given the PowerPC-like nature of the Sony/IBM Cell CPU, which would (if reports are correct) have been an immensely desirable platform for a new era. (Who knows; perhaps whoever owns the Amiga can resurrect it as a Cell workstation?) And as far as x86 chips go, AMD would have probably been a better choice than Intel. Unless, of course, it's the DRM issue again, and Jobs needs to have a black iron prison in place that the MPAA will sign off on.
And so, the last major bastion of diversity in the world of computers falls to the x86 monoculture, and Apple becomes just another PC manufacturer, albeit one with its own OS. Perhaps in five years' time they'll give up on that as well and switch to providing a Cocoa layer over Windows too? After all, it'd save them a lot of hassle.