I received a comment and a few e-mails asking how this was done. Well...I actually wrote this up (along with a sample database) a couple of years ago, so today's show and tell will be a cheat, as I am just going to link you over to that entry. With that, here you go. Enjoy.
SnTT: Cascading Navigation In Notes
Editor's Note: The sample database that is available for download also includes all my experiments using layers to create cascading menus. Remember...Lotus Notes is like an onion...it's got layers! :-)