Comment 34 for bug 105102

With my P4P800 I could not shut down in Edgy or Fiesty (would hang at the end). It turns out that an option for apm had been disabled. You'll need to re-enable it with the following:
from http://ubuntuforums.org/archive/index.php/t-715.html

nano -w /etc/modprobe.d/power

add the following bit of code to the new file:

options apm power_off=1
#optional: may also be added to above: realmode_power_off=1

see also:
http://ubuntuforums.org/showthread.php?t=237264