I tried gnome3 and gnome3-staging ppas to test running Gnome 3.8. After a while I decided that Gnome 3.8 wasn't for me, so I did a ppa-purge of both the ppas. As described in gnome3-staging ppa page, I also did:

$ sudo apt-get purge libpam-systemd
$ sudo apt-get install libpam-xdg-support

The trouble is, I can't mount my external USB device anymore. When I try to mount it as a user, it fails:

$ udisks --mount /dev/sdc1 
Mount failed: Not Authorized

I am logged in an XFCE session, but the same thing happens in a fallback Gnome session, or from a Unity session. Also, in XFCE, "suspend" and "shut down" menus are grayed out.

I can't also open synaptic package manager from XFCE menus (sudo synaptic works).

After a lot of searching, it seems like it is a policykit issue. I see the following in my ~/.xsession-errors:

(polkit-gnome-authentication-agent-1:5805): polkit-gnome-1-WARNING **: Unable to determine the session we are in: No session for pid 5805

PID 5805 doesn't exist. If I try to start polkit-dnome-authentication-agent-1 from an xterm, I get the same error (different PID):

$ /usr/lib/policykit-1-gnome/polkit-gnome-authentication-agent-1
...
(polkit-gnome-authentication-agent-1:15971): polkit-gnome-1-WARNING **: Unable to determine the session we are in: No session for pid 15971

(the ... lines are warnings from GTK about missing css files etc.).

polkitd is running:

$ pidof polkitd
1495

Is there something I am missing?