Comment 16 for bug 238799

I was able to get access via VNC tunneled through SSH by changing the following settings in policykit. You can do it locally via Authorizations, or you can do it remotely using "sudo ck-launch-session polkit-gnome-authorization" in a terminal window in your tunneled VNC session. This worked on Ubuntu 9.04 Server RC running xubuntu-desktop, so as always YMMV.

For system configuration, change all implicit authorizations under org -> freedesktop -> systemtoolsbackends -> Manage System Configuration (org.freedesktop.systemtoolsbackends.set) to "Admin Authentication."

For user management, change all implicit authorizations under org -> freedesktop -> systemtoolsbackends -> self -> Change User Configuration (org.freedesktop.systemtoolsbackends.self.set) to "Authentication."

Reset gdm by rebooting or running "sudo /etc/init.d/gdm restart" from a terminal window, and you should be able to unlock the user settings control panel and other similarly useful things through your tunneled VNC session.