The SFDC Blog had an article on Double Dropdown lists (options on one picklist dependant on the selection made in another picklist) found at http://crmsuccess.blogs.com/crmsuccess/2005/02/dependent_dropd.html. 
(see "Download double_drop_down_example.txt" for code)

I solved a couple of problems I had when a category had no sub category entries, but I'm having trouble solving a problem with the "Update Case Record" function. When I click the "Save" button, nothing happens. I am getting a status value back from the server of 500 rather than 200.

I understand the Summer release in June will have this functionality, but I need it yesterday.

Anyone have a solution?

Anyone have another version of this double dropdown for opportunities or leads (or a list of changes to make it work for opportunities or leads)?