Many thanks to AnnieDog for pointing me to this thread. I had the same non-shutdown problem.
I removed "acpi=ht" from Grub's boot command line and replaced it with "apci=force".
This did the trick and the PC shutdown very quickly
I had been using a Mandrake 10 (also installed) version of Grub (because GRUB in earlier versions of Fedora did not see my SATA drive containing Windows). Mandrake's Grub had contained the following: "acpi-ht" I never gave any thought to what this means.
I discovered that this command was disabling acpi and so I simply removed any reference to acpi from the Grub boot command line. This solved the shutdown problem also and seems to result in faster booting and better performance also. (Perhaps I am immagining this).
Thanks to all who posted to this thread.