I have a similar problem afrer upgrading xfce4 to version 4.8. Immediatly after a fresh boot removable drives are recognized and mounted, both from desktop and thunar but no longer after a new login of the xfce session. In the new session I get 'Failed to mount ..., Not Authorized' in desktop and thunar. I have udisks,gvfs and libpam-ck-connector installed!