It depends what you want in the menu.
If it is WP pages, then use the depth=x parameter to limit the level of pages displayed intially, then set the theme to list another level sub-pages when on a lower level.
http://codex.wordpress.org/Template_Tags/wp_list_pages#List_Sub-Pages_.28Wordpress_2.0.1_or_newer.29
I don’t particularly like that style of menu though as there is quite a long delay between when you click and the next menu item being available. I would be inclined to use Javascript (with HTML fallback) to display the menu items a little faster, I’m not sure how to do that though (my Javascript knowledge is rather feeble).
It doesn’t sound like quite what you are after, but I generally recommend creating a flyout menu with my dropdown menu plugin instead of the approach you are using. Unfortunately I haven’t built in a flyout functionality to the CSS generator for my plugin yet, but I do customisations for a reasonable price if that’s of interest to you.
Thanks Ryan, I already have a drop down menu that I use, the reason I need a vertical drop down is the client has more page parents that would fit on a horizontal menu so I need to go vertical.
Il have a look through the codex, see if i can work something out.
