It looks like this bug has been around for a while. This appears to be a duplicate:
https://launchpad.net/distros/ubuntu/+source/linux- source-2.6.15/+bug/22148 -- k8n system doesn't boot without acpi=off option. then poweroff doesn't work. https://launchpad.net/bugs/71189 -- ubuntu-bugs mailing list [email protected] https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
