Odd ACPI shutdown and power off issue

[Steve]

I'm using 6.2-R on a Dell PE400SC.

I recently enabled "Remote Wake Up" (Wake on LAN) in BIOS.  Sending a
magic packet to the powered down PC will power it up and it boots
normally.  However, now if I attempt a shutdown and power off using
"halt -p" or "shutdown -p now" I get a reboot.  Disabling Remote Wake
Up reverts back to the expected behavior.

I've searched and found the sysctl knob "hw.acpi.disable_on_poweroff"
as a possible solution, but sysctl gives me "unknown oid".

Any ideas?