Put the following in a file called script.sh. This is slightly modified from a script I found on the web. You can find radeontool in the AUR packages.

#!/bin/sh

# if launched through a lid event and lid is open, do nothing
echo "$1" | grep "button/lid" && grep -q open /proc/acpi/button/lid/LID/state && exit 0

# remove USB 1.1 driver
rmmod uhci_hcd

# sync filesystem and clock
sync
/sbin/hwclock --systohc

# switch to console
FGCONSOLE=`fgconsole`
chvt 6
/usr/sbin/radeontool light off

# go to sleep
echo -n "mem" > /sys/power/state

# readjust the clock (it might be off a bit after suspend)
/sbin/hwclock --adjust
/sbin/hwclock --hctosys

# reload USB 1.1 driver
modprobe uhci_hcd

# turn on the backlight and switch back to X
radeontool light on
chvt $FGCONSOLE