Thanks, that works. There is one thing I don't get though. If you want to show a tab to administrators, but hide it from non-administrators, shouldn't it be done by changing this:
<siteMapNode url="~/admin/Pages/Blogroll.aspx" title="blogroll" description="" roles="administrators, editors"/>
to this:
<siteMapNode url="~/admin/Pages/Blogroll.aspx" title="blogroll" description="" roles="administrators"/>
When I make that change the Blogroll tab still shows up for editors.