Let me expand, since it seems I was not clear.
I am unable to access the commands in the System-> Administration menus because the items (in "Services" for example) are shown in gray. Commands from a terminal window (such as "su", "sudo" and "gnomesu") all work as they should. Setting the various options in rc.conf works, as you would expect.
It is simply that these items are not accessible from within Gnome. It would seem to be a permissions issue; in the past gnomesu would open a box requesting the root password. Once entered, the services (for example) could be selected from the menu. This appropriate behavior no longer happens.
I should add that this behavior has come and gone since I started using Gnome back in the 2.4 days. Usually installing gksu cured it, but not this time. Hence my question.