Debian squeeze & wheezy, kernel 2.6.32-4 and later, Xorg 7.5
You should use a file in /etc/X11/xorg.conf.d, e.g. /etc/X11/xorg.conf.d/synaptics.conf, for configuration (but this will work just as well in /etc/X11/xorg.conf). The following example shows how to enable tapping and how to configure various other options; you probably won’t need everything in it. Comment out any old config for synaptics before replacing it with this new config.
Section "InputClass"
Identifier "Touchpad" # required
MatchIsTouchpad "yes" # required
Driver "synaptics" # required
Option "MinSpeed" "0.5"
Option "MaxSpeed" "1.0"
Option "AccelFactor" "0.075"
Option "TapButton1" "1"
Option "TapButton2" "2" # multitouch
Option "TapButton3" "3" # multitouch
Option "VertTwoFingerScroll" "1" # multitouch
Option "HorizTwoFingerScroll" "1" # multitouch
Option "VertEdgeScroll" "1"
Option "CoastingSpeed" "8"
Option "CornerCoasting" "1"
Option "CircularScrolling" "1"
Option "CircScrollTrigger" "7"
Option "EdgeMotionUseAlways" "1"
Option "LBCornerButton" "8" # browser "back" btn
Option "RBCornerButton" "9" # browser "forward" btn
EndSection
via SynapticsTouchpad – Debian Wiki.