Just make sure APM is enabled in your bios, compile the kernel enabling APM
and as a sub-option make sure to enable CONFIG_APM_POWER_OFF. Once you have a
kernel configured like this, on a halt the machine sould power itself off.
This is in the 2.0 series kernel. Things have changed a little in the 2.1
series so a modified halt would need to be used.
Geoff Hoff