... and another way to achieve a similar result :-)
<asp:Menu
ID="Menu1"
runat="server"
>
<items>
<asp:MenuItem text="Home" navigateurl="~/" selectable="true" enabled="true" />
<asp:MenuItem text="Admin" selectable="false" enabled="false">
<asp:MenuItem text="AdminLink" selectable="true" navigateurl="~/admin/Link.aspx" />
</asp:MenuItem>
</items>
</asp:Menu>
... and in your code behind, enable the Admin menu for the correct user type.
MenuItem miAdmin = Menu1.FindItem("Admin");
if (oUser.UserType.ToLower().CompareTo("administrator") == 0)
{
miAdmin.Enabled = true;
}