If you look at the bridges on the chip itself, where the L5's are you'll see something like this;
Early XP's; I::I
Late XP's; I:::
What you need to do, on the later build XP's, is to connect that last set of bridges, so they look like the first example above. That's it. If you've got an early XP, it's already connected (or, actually, AMD didn't cut them).
XP to MP goodness is one bridge away.
Here's an article on the front page where I got the pic attached below;
http://www.overclockers.com/articles468/
Here's a really good article by Lews on unlocking the XP;
http://www.overclockers.com/tips693/
See also the stickies at the top of the AMD topic for more info and tips on unlocking.
Cheers!
B.