This is what the folks suggested at a local gunstore.
http://prostores3.carrierzone.com/servlet/rusteprufe_com/CategoriesI'll agree that WD-40 shouldn't cause rust, but I've seen it happen on two guns. Granted, one was kept in pretty harsh conditions without much TLC, but when I got the guy to switch to Rusteprufe, he stopped having those problems.
Was it a scientific test? No. Could there have been another reason? Sure. But was the issue cured when he switched? Yes.
[This message has been edited by milkman00 (edited 12/6/2011 12:35p).]