Comment 5 for bug 367960

The relevant source code line is:
  QProcess::startDetached( "gdm-signal -h" ); // Gnome shutdown

The first problem with that is that gdm-signal in the "powermanagement-interface" package, which isn't preinstalled in a standard Ubuntu installation.
The second problem is that even if one installs that package, gdm-signal -h seems to no longer work.

A simple workaround is to not install the powermanagement-interface package, but to create a /usr/bin/gdm-signal script instead with the following contents:
#!/bin/sh
gnome-power-cmd shutdown
(don't forget to chmod +x the script)

Unfortunately, gnome-power-cmd is no longer there in Karmic.
Someone should ask the gnome people for a stable way to poweroff without having sudo priviledges and without modifying the sudoers file. :)