Hi, I'm having the KDE authentication problems too. Well actually I had the exact same error as the post above when running KDE 4.6 (installed via SVN) and when I reinstalled a complete fresh system and just pkg_add KDE 4.5.4, I get no dialogs at all when trying to change a system setting, like in the KDM theme changer. Click to apply, nothing at all happens. All I can do is discard the changes and move on basically.
I'm guessing there is something going on with dbus or console/policykit, but I'm not too sure. I think I read a KDE dev on their forums saying one of the problems is the way the FreeBSD port of KDE installs its dbus files to its own prefix instead of standard /usr/local/etc/dbus, etc..
But if this is the case how come some people have no issues and PC-BSD etc run just fine?