Comment 160 for bug 308191

@Nathan: you can do
 xinput set-int-prop "Synaptics Tap Action" 8 "0 0 0 0 1 2 3"
See 'man synaptics' for more info. If you run this from Startup Applications the settings will get overridden by Gnome's (which doesn't appear to have a way to configure this) as it sets itself up. What I have is a script to keep re-applying the settings for 2 minutes:

 #!/bin/bash
 itry=0 ; while ((itry<24)) ; do itry=$((itry+1))
  xinput set-int-prop "Synaptics Tap Action" 8 "0 0 0 0 1 2 3"
  sleep 5
 done

Still I get some config resets at seemingly random times, so I have the xinput command at hand to re-apply them when needed..