At ArsTechnica, here, Jon "Hannibal" Stokes reckons that:
For the real reason behind the switch, you have to look to the fact that it's the iPod and iTMS — not the Mac — that are now driving Apple's revenues and stock price. As I stated in my previous article on the switch, Apple is more concerned with scoring Intel's famous volume discounts on the Pentium (with its attendant feature-rich chipsets) and XScale lines than it is about the performance, or even the performance per Watt, of the Mac.
It's critical to understanding the switch that you not underestimate the importance of Intel's XScale to Apple's decision to leave IBM. The current iPods use an ARM chip from Texas Instruments, but we can expect to see Intel inside future versions of the iPod line. So because Apple is going to become an all-Intel shop like Dell, with Intel providing the processors that power both the Mac and the iPod, Apple will get the same kinds of steep volume discounts across its entire product line that keep Dell from even glancing AMD's way.
and....
The cold, hard reality here is that the Mac is Apple's past and the iPod is Apple's future. It's a shame that Steve Jobs can't be upfront with his user base about that fact, because, frankly, I think the Mac community would understand. The iPod and what it represents ... is the Macintosh of the new millennium. There was no need to put on a dog and pony show about how IBM has dropped the performance ball, when what Jobs is really doing is shifting the focus of Apple from a PC-era "performance" paradigm to a post-PC-era "features and functionality" paradigm.
Comment: For the hard of thinking, it's an opinion piece and it's Jon "Hannibal" Stokes's opinion.