There is nothing inherent to the 2009 CPUs that keeps them from being upgraded. Unless Apple sodered the CPUs to the daughterboard (which I doubt) there no reason can't drop in another CPU. You will piss away your warrantee most likely. But other than Apple not wanting aftermarket socket stuffing, there is nothing inherently prohibiting that.
The CPUs and memory being shoveled onto a daughtercard is Apple's (which other folks have had in other contexts for similar reasons) idea. It isn't inherent in the CPU/memory in a single package design. Apple did it to conceptually make getting at memory upgrades easier. Pull the daughter card out and then zero obstructions to getting to the memory slots.
That actually raises the cost of the finished product and would also be done by making box taller. It is a well reasoned compromise to hold the hieght and just the width in a more clever manner while still maintaining clean accessibility inside.
Ah, so the socket is essentially untouched within the Xeon family? Does this mean, supposedly, that in 2011 when my 2008 Mac Pro's AppleCare expires, I could simply drop in a new processor and "upgrade" the system (minus the other benefits to a completely new system update)?