I dont think it had much to do with PowerPC architecture or pricing or feeling unloved by IBM. In fact, I dont think it had much to do with the actual processor hardware at all.
I think its all about the networking of the immediate future. Let me explain.
The Trusted Computer Group is a multivendor association that grew out of Microsofts pre-emptive Trusted Computing Platform effort. Microsoft realized it couldnt force this down the manufacturers throats, so it formed the TCG to give it the veneer of respectability and "open standards."
The TCG just issued the first version of the Trusted Network Connect protocol. TNC is meant to help stop the spread of viruses, worms, DOS attacks and other common points of vulnerability in computing networks, even wireless ones. Even more recently, implementations of the TNC specification have been announced for RADIUS servers and 802.1x clients.
While the TNC specification is still very young (Version 1.0), it is also the first-ever protocol to attempt to enforce network access on a per-client or per-network basis, which makes it a spec worth exploring if youre doing enterprise wireless development. Or trying to get your machines accepted by those who will use these kinds of networks.
TNC uses a number of ways to authenticate the supplicant trying to join the network, one of which is the hardware Trusted Computing Module bolted on to TCG-compliant computers. Now, heres where the trail starts to heat up. The TCM wont be available on non-Intel machines any time soon, since it uses Intel silicon to implement the functions needed. If you want to play in a TNC-defined network, you need to be on Intel hardware (at least for the near term).
So, Apple looks at this up-and-coming networking protocol that will be supported by Microsofts upcoming Longhorn OS. It sees the advantages it will give to users if implemented correctly. Apple realizes it will be shut out of these networks for years using its current hardware. Apple collectively freaks.
So, combine this possible lockout with the other factors like IBM not ramping up clock speed on the single-core PowerPC line (but doing so for MS multicore Xbox chip), and you have Apple looking around for different hardware.
So, heres Intel watching Microsoft give a chunk of chip business to IBM. They are feeling lonely, no doubt. And along comes Apple looking for hot hardware that will be in sync with where the market will be forced to go in the future. (When your company says the entire network will be TNC-compliant, youve got to go with that, right?) They make a deal.
It may not be pretty, but it sure makes business sense.