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If you want the user to have the ability to shutdown or reboot you have to change file permissions for the halt command. Reboot is a symlink to halt, so that doesn't need to be changed. Type as root:

chmod +s /sbin/halt

Topic: http://bbs.archlinux.org/viewtopic.php?t=2787

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Another option is to do this using sudo. First install sudo:

# pacman -Sy sudo

Then, as root, add the following to the end of /etc/sudoers using the visudo command. Substitute user for your username and hostname for the machines hostname.

user hostname = NOPASSWD: /sbin/shutdown -h now
user hostname = NOPASSWD: /sbin/reboot

Now your user can shutdown with sudo shutdown -h now, and reboot with reboot.

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A third option, under Gnome is to perform the following commands:

su
your password
chmod +s /sbin/halt
cd /var/run
mkdir console
cd console
touch username

This adds the "reboot" and "halt" options to Gnome's logout menu for the user defined.

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A fourth option, under XFCE is to install sudo. Then add the following line to /etc/sudoers using the visudo -f command. For all users:

%users hostname=NOPASSWD:/usr/lib/xfce4/xfsm-shutdown-helper

Substitute hostname for the machine's hostname.

For single user:

user hostname=NOPASSWD:/usr/lib/xfce4/xfsm-shutdown-helper

Substitute user for your username and hostname for the machine's hostname.

This activates the "reboot" and "turn off" options to XFCE's session-manager logout dialog for the user defined.