just put a file in /etc/acpi/events for the battery and ac_adapter ACPI events (if you have laptop-mode-tools installed you can add the action lines to the two files already there called lm_ac_adapter and lm_battery):

event=battery.*
action=/etc/acpi/actions/brightness.sh battery

event=ac_adapter
action=/etc/acpi/actions/brightness.sh ac

and then put into /etc/acpi/actions/brightness.sh something like:

#!/bin/bash

# levels are from 0 (very dark) to 7 (very bright)
BATT_LEVEL=0
AC_LEVEL=7

case "$1" in
        battery)
                echo "level $BATT_LEVEL" > /proc/acpi/ibm/brightness
                ;;
        ac)
                echo "level $AC_LEVEL" > /proc/acpi/ibm/brightness
                ;;
esac