As long as someone else has revived this thread......
The upcoming Nehalem architecture will have up to 8 cores on a chip. And HyperThreading returns. That means 16 threads per chip.
A dual-socket system will be able to execute 32 threads. Holy crap. Right now, a 32-thread machine would be considered a supercomputer. Heck, 16-thread machines are considered "big iron".... And it's coming to a "workstation" class machine later this year. (Notebooks will be getting 8-thread chips!)
I was planning on upgrading my desktop with the next iMac iteration, but now I am certainly going to wait for a Nehalem iMac. I mean, this will be nearly as big an upgrade as the PPC to Intel upgrade. (For everything but the Mac Pro, it absolutely will be, for the Mac Pro, it will be close.)
I'm still hoping Apple will start using desktop CPUs and chipsets in the iMac. I would have gotten an iMac on its last upgrade if they had used a quad-core chip in it. (I just upgraded my PC with a Core 2 Quad 9450, a 2.66 GHz chip, and it is a screamer. Too bad the chipset isn't OSx86/Hackintosh-okay, I'd love to use it for my movie editing.)