[CentOS] Hibernating a laptop with the power button (Thinkpax X31) under CentOS 5.4

I am having a weird problem trying to get hibernation to work when I press the power button on my Thinkpad X31 under CentOS 5.4.

I've modified /etc/acpi/events/power.conf to contain:

# ACPID config to power down machine if powerbutton is pressed, but only if # no gnome-power-manager is running

event=button/power.* action=/bin/ps awwux | /bin/grep gnome-power-manager | /bin/grep -qv grep || /sbin/shutdown -h now

And restarted acpid (sudo /sbin/service acpid restart).

I don't run gnome, so there won't be a gnome-power-manager process running.

When I press the power button nothing happens (acpid does log that it ran the above action).

If I run the action line above in a shell (as root), the machine does hibernate just fine.

What is going on here?