The Wilson guys (Wilson Combat, 45 autos) use it by the 50 gallon drum full. There was a pump spray bottle of WD-40 sitting in the background on a bench in a photo of Wilson's custom shop in a magazine article I read a couple years ago.
WD-40 works fine. I agree with IDhunter--it's not the best for lubing something, but it does Displace Water (that what WD stands for, "water displacing") and it doesn't run off of surfaces very easily.
It will attract dust. But so will many other gun lubes on the market. All oils tend to attract dust. If dust bothers you, use some Johnson's Paste Wax to seal the metal against moisture...
I've used WD-40 for a long time on my guns, and just like the guys above, I haven't had any rusting or "gumming" up.
Dan