The Core i7s on Newegg won't work. You need a Xeon class processor for the Mac Pro still. Intel doesn't build the logic into the consumer CPUs to support talking to another physical chip, since in most cases it would be wasted die space.

And until the systems start shipping and people tear into them, noone will really know what is involved in upgrading them. Photos from Apple.com show the processor and memory on a removable board, so they may just plug in a single processor only board into the models with one CPU.

Also one correction above, the base price on the old 8 core systems was $2799, and removing one processor brought it to $2399. I may have been remembering my corporate discount prices instead of the retail prices.