On 2 completely different machines I have the same problem after upgrading from 13.04 to 13.10: All programs/task requiering sudo permissions (software updater, starting synaptic, installing programs, shutdown) do not start / execute. No gksu (or other) window comes up. Synaptic never shows, software updater pops up with a "you are not allowed to perform this action" message.
All programs / task work when started from terminal with sudo, like "sudo synaptics" asks for my password, starts and works as expected.
I am in the admin group which is liked in sudoers config (verified with visudo and getent group admin).
Some link from GUI to gksu (or similar) is broken.