What on earth do you need all that power for? Eight cores are of course better than four, but the difference really is quite marginal for today's computing applications.
I'd kill for an 8-core system when doing my 3D rendering. I've been debating picking up a scad of Mac Minis to create a render farm, but if there were 8-core Mac Pros available I could just get one of those instead.
My first computer (with a real user interface) was a toshiba laptop with an external floppy drive, windows 1995 and 300mb hard drive! I think the orignal price tag was like $7500..
I am planning on upgrading the the 8 core system when it comes out. My G5 quad as fast as it is, sometimes is too slow for what I do.