hi,
the xfce version in debian squeeze is quite old and I suggest you try a more recent one as xfce 4.10 is awesome and solves many issues. anyway, this should be connected to 'xfsm-shutdown-helper->shutdown()'. Check if you need to be in the 'power' group and/or if hal/dbus is running...
also, is there anything logged in the log files? if you add something like:
Code:
reuben ALL = (root) NOPASSWD: /usr/sbin/xfsm-shutdown-helper
to your `visudo` (/etc/sudoers) does it help?