There is no hardware switch or function key combination for turning off the touchpad. Stoichyu is right--there was some edit in xorg.conf and I think something in /dev. Currently, the mouse lines in xorg.conf are:
Option "Protocol" "auto"
Option "Device" "/dev/mouse"
Option "ZAxisMapping" "4 5"
Mouse0 is the USB mouse. /dev/mouse is a symlink to /dev/input/mice. That contains event0, event1, event2, event3, mice, mouse0, mouse1. The touchpad is mouse1.
I think the old solution involved a symlink to something and maybe a change from "Protocol" "auto" to something else. Does this ring any bells? I'd like to avoid tpconfig if possible.
I read quite a few messages in this that were turned up in a Google search, but none of the suggested solutions matched what I have in xorg.conf.
I hope in the future we can have a failsafe way to back up the entire message board. We've lost some crucial information every time some evil has befallen the board.