Created attachment 3073 
dbus-monitor output

I'm having the same problem as the Debian maintainer here:

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=582305
http://foo-projects.org/pipermail/xfce/2010-May/027130.html

pkaction --verbose:

org.freedesktop.upower.suspend:
  description:       Suspend the system
  message:           Authentication is required to suspend the system
  vendor:            The UPower Project
  vendor_url:        http://upower.freedesktop.org/
  icon:              system-suspend
  implicit any:      no
  implicit inactive: no
  implicit active:   yes

org.freedesktop.upower.hibernate:
  description:       Hibernate the system
  message:           Authentication is required to hibernate the system
  vendor:            The UPower Project
  vendor_url:        http://upower.freedesktop.org/
  icon:              system-suspend
  implicit any:      no
  implicit inactive: no
  implicit active:   yes

ck-list-sessions:

Session8:
	unix-user = '1000'
	realname = '(null)'
	seat = 'Seat1'
	session-type = ''
	active = TRUE
	x11-display = ':0'
	x11-display-device = '/dev/tty7'
	display-device = '/dev/tty1'
	remote-host-name = ''
	is-local = TRUE
	on-since = '2010-08-11T06:46:32.007743Z'
	login-session-id = '4294967295'

upower --dump:

Daemon:
  daemon-version:  0.9.5
  can-suspend:     yes
  can-hibernate    yes
  on-battery:      no
  on-low-battery:  no
  lid-is-closed:   no
  lid-is-present:   no

For example gvfs and thunar-1.1.1 is able to find disks and mount them, udisks --mount is able to mount devices, and upower --dump is reporting the above "can-suspend: yes" and "can-hibernate: yes" but the buttons for hibernate/suspend are greyed out in xfce4-power-manager's system tray icon.

If I compile xfce4-power-manager without policykit, they show up.

The attached file is dbus-monitor output from time to xfce4-power-manager start, to it's closure.