i searched a lot for a solutions to this problem
i have an two Ubuntu 12.04 installs both have the same issue , after invoking a shutdown with varied ways sudo init 0, "sudo halt" , menu shutdown, but they all have the same result,
all goes good until it reaches

Will now halt 

[time] power down

then it hangs there forever, i tried to modify "/etc/default/grub"
adding GRUB_CMDLINE_LINUX="acpi=force" then tried GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi=force" and updating grub each time in vain
note that restarting the machine is not affected, because it reboots smoothly
I'm using GPT/UEFI install for x64 12.04 both,
and in the grub added this entry which also hangs

menuentry "System shutdown" {

    echo "System shutting down..."

    halt

}

any idea of how to fix this would be appreciated.

UPDATE

/var/log/syslog is empty before and after shutting down-starting

note that using a direct restart (like sudo reboot) works perfectly, but shutting down and starting manually is faulty