Menu and selected items

[JJ]

I have a menu that only displays the top (static) level. I want to display a
menu item as 'selected' not only when it is clicked on ,but when a user is
accessing any page that is below that main pages' heirachy in the sitemap
(which is the datasource for the menu).

i.e. I want to have the parent item in the menu selected when a child page
(based on the sitemap heirachy) is being accessed.

Do I parse the sitemap file (note that I am using security trimming if that
makes a difference) or is there an eaiser/more practical way?

Thanks,
JJ