The following was published to a public Apple mailing list:
I attended a briefing today on IBM's high performance computing technology, which is hinged on their Power4 CPU (this CPU has awesome performance in the various real-world benchmarks I have seen). After the briefing, I asked the presenter (a chief engineering manager from IBM) about the Power4 derivative for desktops and low end servers to be announced in October. You may recall that there has been speculation that this CPU would find its way into PowerMacs in the future. Well, it sounds like this CPU is not in Apple's future -- the "over 160" vector instructions are not AltiVec (even though AltiVec has 162 instructions), and there are technical issues that would prevent AltiVec from ever marrying with Power4 or its successors. Furthermore, the guy came right out and said that they have pitched the desktop Power4 to Apple, but Apple was not interested.
So, although Power4-based PowerMacs seemed like a promising (and likely) possibility, it looks like it won't happen. I guess we will have to wait and see what Apple has in store for the future . . . .