Oh, bother - this will never end :)
The new kernel in the new Slackware has yet again a different way to configure ACPI. First, edit /etc/rc.d/rc.acpid and enable logging, by appending -l to this line:
/usr/sbin/acpid -l
/var/log/messages when you press butons.event=ibm/hotkey HKEY 00000080 00001004 action=/usr/local/sbin/acpistandby.sh
/usr/local/sbin/acpistandby.sh:# everything before this point is sleep echo mem > /sys/power/state # everything after this point is wakeup