Da weiß ich auch nicht weiter, sollte so eigentlich funktionieren, zumindest tut es das bei mir
Section "InputClass"
Identifier "touchpad catchall"
Driver "synaptics"
MatchIsTouchpad "on"
Option "TapButton1" "1"
Option "TapButton2" "2"
Option "TapButton3" "3"
Option "VertEdgeScroll" "on"
Option "HorizEdgeScroll" "on"
# This option is recommend on all Linux systems using evdev, but cannot be
# enabled by default. See the following link for details:
# http://who-t.blogspot.com/2010/11/how-to-ignore-configuration-errors.html
MatchDevicePath "/dev/input/event*"
EndSection