Nonetheless, there is ample anecdotal evidence that WD-40 is hydroscopic, no matter what the manufacturer claims about it. It is wonderful for releasing frozen bolts, but steel that has been sprayed with WD40 and not relubed afterwards will rust much faster than had it simply been left dry to begin with.
If what you are looking for is a good spray-on rust preventative, then the Birchwood-Casey "Barricade" stuff is really good. Like WD-40 it contains both a solvent and lubricating oil, so you spray it on and it spreads out and penetrates every nook and cranny, but can be left on the steel and will actually protect the steel instead of attracting rust. It's just not any good at releasing stuck bolts...