Well, since we already know the C2D MacPros will fit the upgraded 4 core chips, at least based on the design of the chip, it is possible. Intel is not doing anything special with the chips for Macs, but that doesn't mean Apple would not put some funky connector or something in there to stop you from upgrading. Unless their is some motherboard firmware update that Apple doesn't release to the public you may be able to. Then again, who knows?
As far as I have read around many places, any alterations to the core hardware of a Mac usually results in tears.
What do you do that you actually can harness 16 cores?
We'll, Can you put the new quad-core chips in a G5? If you can than when the 8-core processors come out Mac Pro's motherboard will most likely support them.
I know a site have successfully upgraded a 4 core Mac pro to 8 core with no problems whatsoever.
You should know, however, that 8 core is not due out for 4 or 5 years
4 or 5 years? Where have you been? in 3 years Intel will be releasing an 80-core chip for desktops.