Two reasons for using unsalted butter:
(1) You have total control of the salt content in the recipe - you never know how much salt has been added in salted butter.
(2) Salt, in addition to being a preservative, can "mask" the smell of butter that has "gone off" flavor wise. This goes back what GB said earlier. With modern refrigeration and expiration dates on the packages - I really doubt you'll notice much difference between salted and unsalted butter in this regard.
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain