Here is my experience installing the software for a Synaptics touchpad on a Compaq Presario 1690 laptop. Note that this is based on a fresh Debian 3.1 testing install with a 2.6.10 kernel from unstable. Some config file lines may differ from your setup:

1. Using apt or the Synaptic Package Manager (not to be confused with the Synaptics touchpad software), install "xfree86-driver-synaptics". Then read and follow the package's README.Debian file since it is likely more up to date and correct than this document.

2. I recommend you install "qsynaptics" as well, or perhaps "ksynaptics" if you prefer; in this case I am using qsynaptics.

3. Open a terminal, "su" to root, and open "/etc/X11/["XF86Config"]-4" in your text editor.

4. Add/replace in the "?InputDevice" section for the touchpad the following lines:

 Section "InputDevice"
  Identifier    "Synaptics Mouse"
  Driver        "synaptics"
  Option        "Device"        "/dev/psaux"
  Option        "Protocol"      "auto-dev"
  Option        "LeftEdge"      "1700"
  Option        "RightEdge"     "5300"
  Option        "TopEdge"       "1700"
  Option        "BottomEdge"    "4200"
  Option        "FingerLow"     "25"
  Option        "FingerHigh"    "30"
  Option        "MaxTapTime"    "180"
  Option        "MaxTapMove"    "220"
  Option        "VertScrollDelta" "100"
  Option        "MinSpeed"      "0.09"
  Option        "MaxSpeed"      "0.18"
  Option        "AccelFactor"   "0.0015"
  Option        "["SHMConfig"]" "on"
 #  Option      "Repeater"      "/dev/ps2mouse"
 EndSection

5. Add/replace these lines to the "?ServerLayout" section:

      Section "ServerLayout"
      ...
              InputDevice    "ConfiguredMouse" "CorePointer"
              InputDevice    "Generic Mouse"   "AlwaysCore"
              InputDevice    "Synaptics Mouse" "AlwaysCore"
      ...

6. Save the file and restart X (Ctrl+Alt+Backspace). (You may have to reboot, as well. I don't know why, but it didn't work for me until I rebooted, and I'm not knowledgable enough to know what to do to make it work without rebooting.)

7. Log in to your GUI and try running qsynaptics or ksynaptics. If all is well, qsynaptics should report that all is well and you should be able to configure your touchpad.

(Thanks to Craig Fields of houseofcraig.net for his assistance.)

8. Firefox has the default configuration to interpret horizontal scrolling as back or forward button events. If you don't want this you can disable horizontal scrolling via qsynaptics or ksynaptics, but if you want to keep horizontal scrolling (e.g. for viewing large images) do the following:

(This is not stricktly about touchpad configuration, but there is a lot of wrong information on that configuration on google, so I thought it would be useful here.)