Is there a plug-in out there that Responds Wordpress Custom Drop-Down menus?
Ideally similar to how it's done here: http://suan.github.com/make_dropdown/demo.html
But I'd take what I can get.
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There are plugins out there that implement dropdown menus such as Superfish Dropdown Menu Widget which implements my favorite dropdown styles/script.
BUT, this is really theme territory. If your coding isn't that strong, this may be out of reach, but if possible I'd encourage you to implement this in the theme. There are tutorials out there to implement the same script as part of your theme.