If you ever wondered how your window manager can shutdown or reboot your computer without asking for a root password, then the most likely answer is dbus.
This command will reboot a machine running dbus:
dbus-send --print-reply --system --dest=org.freedesktop.Hal \ /org/freedesktop/Hal/devices/computer \ org.freedesktop.Hal.Device.SystemPowerManagement.Reboot
You can replace “Reboot” with “Shutdown” to power off the machine completely.