I had a similar problem with my Shuttle ak31 v2 motherboard. First off, this motherboard must not handle ACPI correctly. Ever since FC2 I've had to use "acpi=off" in the grub.conf which disables ACPI in Linux, but everything worked fine (it uses APM instead) in FC2, FC3, FC4 and FC5. Once I upgraded to FC6, however, Fedora would no longer turn off the power on my PC when I shut it down. It would just stop, with the last message on the screen being "System halted.". After much googling I found this page, http://mandrivausers.org/index.php?showtopic=38210
which recommended adding "acpi=off apm=on apm=power-off" to the grub.conf. Since my acpi was already off, and I new my APM was already on I simply had to add "apm=power-off" to my grub.conf and now Linux will shut the power off on my PC after it shuts down!
My relevant grub.conf entry now looks like this:
title Fedora Core (2.6.19-1.2895.fc6)
kernel /boot/vmlinuz-2.6.19-1.2895.fc6 ro root=/dev/hda2 selinux=0 vga=791 acpi=off apm=power-off elevator=anticipatory
Hope this info helps.