There have been a lot of postings on SIGIA with regard to Jakob Nielsen's kind of bland article on information foraging recently - but the article itself did send me off in search of some information retrieval articles. One I found called "Web Page Design: Implications of Memory, Structure and Scent for Information Retrieval" by Kevin Larson and Mary Czerwinski at Microsoft research had some interesting conclusions (and I'll be honest, I was naughty, and skim-read the body, jumping to the conclusions). The conclusions are great because they suggest that if you ensure that the "information scent" (to use the in "meme" du jour) at each classification level is strong - i.e. label things clearly such that they give you a clear idea of what they are, and what else you'll find if you follow the link; and aim for a middle-path between a broad and shallow in terms of your site structure.
Not rocket science, but it's nice to have a justification for what seems like good practice.