I'm using XFCE with LXDM as login manager in Fedora 16.
One feature of xfce/xscreensaver is the ability to log in as an alternative user while one is already logged in. In XFCE desktop, a "User Switching" widget does this, and in xscreensaver (while locked), there's a button "New Login". Pressing either one shows this message on the desktop: "The name org.gnome.DisplayManager was not provided by any .service files"
Note: while in xscreensaver, button becomes disabled and apparently nothing happens - but if you unlock the screensaver, the messages are visible on the desktop