Comment 36 for bug 119308

This works for me as on my post dated 2007-08-23. Take caution on changing system files:
Do sudo gedit /etc/modules. Add the following line, then save:
apm power_off=1
do sudo gedit /boot/grub/menu.lst (where the l is a lower case L). Find the line that start with "kernel" and add this onto the end, no carriage return, all on the same line which probably wraps over:
acpi=off apm=power_off
then save. Power off however you can, power back up and on "Shutdown" it should. Mine did.
Jerry