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

This will allow you to see acpi events in /var/log/messages when you press butons.

The instructions for the power button are the same as the original ones, but for sleep (Fn+F4) you have to change the event. New /etc/acpi/events/standby:
event=ibm/hotkey HKEY 00000080 00001004
action=/usr/local/sbin/acpistandby.sh

And the script /usr/local/sbin/acpistandby.sh:
# everything before this point is sleep
echo mem > /sys/power/state
# everything after this point is wakeup

Note that removing the usb modules isn't needed any more.