Hi,
If you notice on this site the site menu only shows level 1 and 2, it uses UseTreeView="true" and in theme.skin we have:
<portal:mojoTreeView runat="server" SkinID="SiteMenu"
...
MaxDataBindDepth="1"
/>
MaxDataBindDepth 1 makes it bind only 1 level below the root level.
Hope that helps,
Joe