This is from one of my friends at Apple.
"Is it possible to upgrade the processor? Probably. It'll be difficult though. Laptops are especially tricky. You may need a new logic board. Apple doesn't allow processor upgrades under warranty, so if you try and fail, Apple will do absolutely nothing to help."
From the schematics I've seen, the processor seems to be set into the motherboard and not made to be easily removed. So there is a danger in replacing it as you might damage the logic board on its removal.
So, final answer? It can be done.
Just not easily.
And it could prove to be a very expensive mistake if it isn't done right.