>Well, what if some categories have been deleted and some added?
I'm not sure what you mean. If the ODP adds categories, you're under no obligation to link to them if you don't draw from them; if you add categories, a reference to the (ODP) parent category would be just fine (if you drew data from it). If you add a new category with no ODP content, there wouldn't be any requirement to link back. If you rename an ODP category, no problem, just point back to the pre-renamed-ODP-name.
If you delete categories, no problem -- you did, in fact, delete all except your local categories (and most other licensees pick and choose also.)
If the ODP deletes (more often: moves) a category, your link will be broken. The ideal would be to point to the new category -- that's the kind of service WE try to provide with OUR links. But I don't think the copyright police are going to cross the border over a broken link (or a link removed because it was broken.) We aren't the RIAA.
All of this seems obvious to me, so...I'm guessing you're talking about some other scenario. Could you give an example?