One of my Search Engine College students has written an excellent post titled Can Drop Down Menus be Search Friendly as well as User Friendly?
In it, Lameesh talks about the benefits of nested navigation menus, but the drawbacks of using JavaScript for these, because of their search engine incompatibility. Instead, Lameesh suggests creating a horizontal drop down menu using HTML and CSS with all the nav items created as text links . Nice workaround!