We have a drop down menu and from my perspective, it has kept search engines from diving deep into our site. Once we posted a comprehensive site map (and I mean comprehensive), search engines could get to the content. Then, I submitted the search engines to search engines for spidering just like you would your home page. Very good results for me. However, I am completely dependent on spiders seeing this site map. If they don't, or if I don't submit it to them, they will never get deep into our site. Hope that helps. By the way, we are revising out site and getting rid of all drop down menus.
Free website menus for non profits and free licenses for Open Source Projects