try adding
Section "InputDevice" Identifier "Synaptics Touchpad" Driver "synaptics" Option "SendCoreEvents" "true" Option "Device" "/dev/psaux" Option "Protocol" "auto-dev" Option "HorizEdgeScroll" "0" EndSection
to your xorg.conf. That's what was used by default before.
try adding
Section "InputDevice"
Identifier "Synaptics Touchpad"
Driver "synaptics"
Option "SendCoreEvents" "true"
Option "Device" "/dev/psaux"
Option "Protocol" "auto-dev"
Option "HorizEdgeScroll" "0"
EndSection
to your xorg.conf. That's what was used by default before.