WD-40 is, as has been said, a water dispersant and rust preventative, not a lubricant. The fact that it has some slight lubricating proerties has led more people astray than I can count. I used to work at a bike shop. I've seen what WD-40 does to a chain.
The stuff was actually developed for coating ICBM bodies, to keep water from condensing on them and causing corrosion. It was the 40th formula the company tried. Last I heard, it was still used for that purpose. They apply it with a mop.
Bottom line, use it for what it's for... which is NOT lubricating stuff.
--Shannon