I just fix the same problem on my machine using "apci=ht" in grub kernel line. I'm using an all AMD system....don't know if ht means halt, hyper threading or something else. Try it and see if it works for you.
I tried adding the option earlier and had no positive results. I did grab the kernel for FC2 from the OS directory and installed this version. The results were that the computer would shutdown and poweroff with the kernel-2.6.5-1.358. This isolates the problem specifically to the kernel and not to other operating system functions. Also, since my computer is not that old and the BIOS was upgraded with a version from April 2004, the code in BIOS should conform to whatever caused the breakage for ACPI power-off. An option for acpi=power-off-legacy should be added to the kernel, if these BIOS settings do not conform to the new linux kernel or industry standards. I guess HP should unbreak the BIOS for acpi power-off also.Thanks for the acpi=ht suggestion. acpi power-off is all that is noticably broken. We need some option that addresses this problem specifically.
Thanks,
Jim
