Has no SIMD unit.
Hella huge & expensive, 417 mm2 vs 121 mm2 for .13 PPC970 & 60mm2 for .90 PPC970FX
Slower for our purposes.
So you'd have a slower tower that was more expensive.
Plus the POWER4 is built for high-end servers. The chip tolerances are way beyond what is needed for desktops, and thus it again is very expensive. (The PPC 970FX is built to have similar tolerances to an Opteron or Xeon, both of which are not anywhere near as big-iron-ish as the POWER4.)
A POWER4 Power Mac makes no sense whatsoever. A POWER4 doesn't even make sense for Apple servers, since Apple isn't in the high-end server market. The closest thing they'd market for now is clustered low-end servers - ie. the G5 Xserve in clusters.
Also, any POWER5 derivative for future Macs, even if dual core, will still require a substantial redesign and the addition of Altivec SIMD.
No, But it would be nice to see a single G5 2Ghz at a much better price point.
Probably we'll get that after the next several weeks I think, with 2.5 GHz at the top end. But who knows? Apple is too secretive about their roadmap.