If your sub menu is separate to the main menu and not part of the menu, such as in the red leaf skin. Then yes you end up setting this up by creating different skins for each main menu item. - You don't need to create a skin for every single page, just each main menu item.
This isn't a great method as it involves a lot of work, and I've heard that the snapsis css nav menu handles elements like this much better.
I haven't experimented with a solpart look CSS menu as the HouseMenu styles that I originally tried did not work correctly in IE7 and I haven't had the need to do it since. - Again though, I think the snapsis menu has this covered - have you experimented with it yet? - There is a
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