I don't think nested category is a good idea, by no means.
Even multiple category which is a very good accessory in a photoblog does not atracts me. Algthough it's a good thing, I don't use it because it needs several modifications. Think if you have to submit your photo to a photo competition, what would your selected category be if you only had a single choice? decide!
I think the problem is decision making about choosing a category and nothing else!