I'd try adding """ ALL ALL=(root) NOPASSWD: /usr/sbin/xfsm-shutdown-helper """ to /etc/sudoers. I vaguely remember having a similar problem on a box that had lived from sarge to squeeze and adding that line solved it. However, another machine with a fresh install of squeeze works fine without any manual tweaks and nothing similar in sudoers, so I'm not sure how it should be solved properly.