Comment 7 for bug 591129

I have this problem too.

Running fresh Lucid install with all recent patches on. Kernel: 2.6.32-22-generic #36-Ubuntu SMP Thu Jun 3 19:31:57 UTC 2010 x86_64 GNU/Linux.

The above does not work for me. However, if I start /usr/lib/policykit-1-gnome/polkit-gnome-authentication-agent-1 from a terminal window, I get:
--------------->>
gmos@roadrunner-u:~$ /usr/lib/policykit-1-gnome/polkit-gnome-authentication-agent-1

(polkit-gnome-authentication-agent-1:3748): GLib-GObject-WARNING **: cannot register existing type `_PolkitError'

(polkit-gnome-authentication-agent-1:3748): GLib-CRITICAL **: g_once_init_leave: assertion `initialization_value != 0' failed
<<----------------
and while I leave /usr/lib/policykit-1-gnome/polkit-gnome-authentication-agent-1 running the Software Center runs as expected.

Some more info.
Running Software Center from a terminal window gives:
--------------->>
gmos@roadrunner-u:~$ /usr/bin/software-center
/usr/share/software-center/softwarecenter/apt/aptcache.py:40: GtkWarning: gtk_container_add: assertion `GTK_IS_CONTAINER (container)' failed
  gtk.main_iteration()
<<----------------
And pressing install for some application then adds:
------------------>>
WARNING:root:_on_trans_error: org.freedesktop.PolicyKit.Error.NotAuthorized: ('system-bus-name', {'name': ':1.56'}) is not authorized: org.debian.apt.install-packages
** Message: console message: undefined @1: ReferenceError: Can't find variable: showProgress
<<-------------------

Also tried running Software Center from a terminal with sudo. This gives:
--------------------->>
gmos@roadrunner-u:~$ sudo /usr/bin/software-center
[sudo] password for gmos:
/usr/share/software-center/softwarecenter/apt/aptcache.py:40: GtkWarning: gtk_container_add: assertion `GTK_IS_CONTAINER (container)' failed
  gtk.main_iteration()
<<--------------------
but installations work fine after this point.

Hope this helps solving the problem.