I was buse investigating when items in a menu are shown and it seems that no 'roles' attributes are needed in a Web.sitemap siteMapNode node if you wanted to show nodes only for users with certain roles. The following Web.sitemap will show items 'Admins only 1' and 'Admins only 2' for belonging to 'Administrators' :
<?xml version="1.0" encoding="utf-8" ?>
<siteMap xmlns="http://schemas.microsoft.com/AspNet/SiteMap-File-1.0" >
<siteMapNode url="xx" title="xx" description="xx">
<siteMapNode url="Secured/Administrators/WebForm1.aspx" title="Admins only 1" roles="administrator"/>
<siteMapNode url="Secured/Administrators/WebForm2.aspx" title="Admins only 2"/>
</siteMapNode>
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