I know the ODP copyright statement specifically mentions directory structure. They state "[editors] hereby assign to Netscape the copyright in any material (whether link descriptions, email, communications, directory organization, or otherwise) "
Copying another directory's taxonomical structural is probably not a good idea. Finding out how similar you could make a directory without running the risk of being sued is something you probably need professional legal advice to answer. Given this board's TOS I wouldn't hazard to provide any advice on that.