Seriously difficult.  The process involved in attaching it is an oven which melts all of the solder at once.  To remove the solder, you will need a special iron and then it can still be tricky.  The laptop versions of the C2D are also very expensive.  There is a couple of companies that replace G4s with faster ones in the old models, I wonder if they could do a one-off for ya.  Sonnet and Daystar both do it, but you will probably have better luck with Daystar as they have a smaller operation.

My advice is still to sell you Book and buy a new one.  If you decide to go for the mod though, post your progress to let us know how it goes.