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 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.)