I am using xubuntu, but I am pretty sure that other variants are affected too.

My problem started with being unable to start synaptic from the menu.
So far, I have found out that synaptic is started with pkexec.

When I test starting
pkexec /usr/sbin/synaptic
in a terminal, it asks for the password in text mode.

With another test
pkexec --disable-internal-agent /usr/sbin/synaptic
in the terminal, it says it has not found any authentication agent.

I have two AMD64 PC's and a i386 laptop. The problem occurs on one of the PC's and the laptop.
Comparing the /etc directory trees of the PC's showed many differences, but I could not find anything suspicious about the polycikit configuration.

For the moment, using gksu instead of pkexec is a workaround.

Thanks and Regards,
Markus

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: policykit-1 0.104-1
ProcVersionSignature: Ubuntu 3.2.0-22.35-generic 3.2.14
Uname: Linux 3.2.0-22-generic x86_64
ApportVersion: 2.0-0ubuntu4
Architecture: amd64
Date: Sun Apr 8 17:05:34 2012
InstallationMedia: Xubuntu 12.04 LTS "Precise Pangolin" - Beta amd64 (20120330)
ProcEnviron:
 LANGUAGE=de_DE:en
 TERM=xterm
 PATH=(custom, no user)
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: policykit-1
UpgradeStatus: No upgrade log present (probably fresh install)