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".

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 reboot. (You may be able to modprobe the synaptics module and restart X instead of rebooting, but this is how I did it.)

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.