I know Sye made a guide about hiding diapers but my thoughts are that if everyone posts where they hide their stuff, at least someone will have a working idea for someone, as for everyone the situation is different.
Not that I have diapers at the moment but when/if I need to hide stuff I have a cupboard which has some unused space under it that you can't see unless you get the whole drawer out.
<edit>I made a section in the wiki page How do I hide diapers? where you can post it. (you can sign it with ~~~~~~~~ (8 times ~))</edit>