$ reboot
-bash: reboot: command not found
$ /sbin/reboot
reboot: must be superuser.

In KDE / GNOME, you can just click reboot in the menu and your session sends a message to display manager, which runs as root and does the dirty job of rebooting or shutting down your machine. Unfortunately, things are not that easy if you are using a simpler window manager or want to shutdown from a script. Sure, there is sudo, but that has to be configured first to work without password. Luckily, after a couple of hours of googling (only to find helpful advice as “this can be done easily with ConsoleKit or hal”), reading documentation of dbus, {Console,Policy}Kit and hal and clicking in qdbusviewer, I finally found a way with hal+dbus:

$ dbus-send --system --dest=org.freedesktop.Hal \
  --type=method_call --print-reply \
  /org/freedesktop/Hal/devices/computer \
  org.freedesktop.Hal.Device.SystemPowerManagement.Reboot

To shutdown, just replace Reboot with Shutdown. I’m going to add this to the default icewm configuration, so that the logout menu finally works.