Which desktop do you use. There usually is a powersave applet for that. A generic method could be to use an X screen saver which might be configurable for that. I'm not certain if the "DPMS" section of xorg.conf if just for the monitor, but if you have an external monitor you will want to read up on this section's options: "BlankTime", "StandbyTime", "SuspendTime" and "OffTime".
You may find this ACPI action script written for the thinkpad a useful guide:
So Google terms you can search for are Linux DPMS ACPI "action script".