xfce-polkit opens a pop-up window to enter
the root password when applications such as gparted,
network-manager, synaptic, thunar etc require authentication.

xfce-polkit needs to be enabled to be effective.

From the Applications menu select Settings,
then "Session and Startup".
From this menu, select "Application AutoStart",
and enable "XFCE PolKit".

The authentication agent will become active after the next login.

See http://www.freedesktop.org/wiki/Software/PolicyKit for lots of
documentation, mailing lists, etc. about PolicyKit.
