****EDIT****
These instructions are no longer needed for me. I don’t need to install Gsynaptics to disable my touchpad. I can now simply click System > Preferences > Mouse, click the “Touchpad” tab and uncheck “Enable Touchpad. The touchpad stays disabled, even through reboots. I’m leaving the original post here in case this doesn’t work on your system.

**** Original Post ****
In order to disable your Synaptics touchpad on previous releases of Ubuntu, you would set “SHMConfig” to “true” in xorg.conf and then install the program gsynaptics to disable/enable the touchpad.

The xorg.conf file is different in Intrepid Ibex, there is no section for the touchpad anymore. Now to enable SHMConfig you have to do it via HAL:

Edit this file:
/usr/share/hal/fdi/policy/20thirdparty/11-x11-synaptics.fdi

Find the section that starts with:
<match key="info.product" contains="Synaptics TouchPad">

And add this line to that section before :
<merge key="input.x11_options.SHMConfig" type="string">On</merge>

The file should end up looking something like this:

<?xml version="1.0" encoding="ISO-8859-1"?>
<deviceinfo version="0.2">
  <device>
    <match key="info.capabilities" contains="input.touchpad">
      <match key="info.product" contains="Synaptics TouchPad">
        <merge key="input.x11_driver" type="string">synaptics</merge>
	<!-- Arbitrary options can be passed to the driver using
	     the input.x11_options property since xorg-server-1.5. -->
	<!-- EXAMPLE:
	<merge key="input.x11_options.LeftEdge" type="string">120</merge>
	-->
	<merge key="input.x11_options.SHMConfig" type="string">On</merge>
      </match>
      <match key="info.product" contains="AlpsPS/2 ALPS">
        <merge key="input.x11_driver" type="string">synaptics</merge>
      </match>
      <match key="info.product" contains="appletouch">
        <merge key="input.x11_driver" type="string">synaptics</merge>
      </match>
      <match key="info.product" contains="bcm5974">
        <merge key="input.x11_driver" type="string">synaptics</merge>
        <merge key="input.x11_options.LeftEdge" type="string">0</merge>
        <merge key="input.x11_options.RightEdge" type="string">1280</merge>
        <merge key="input.x11_options.TopEdge" type="string">0</merge>
        <merge key="input.x11_options.BottomEdge" type="string">800</merge>
        <merge key="input.x11_options.ClickFinger1" type="string">1</merge>
        <merge key="input.x11_options.ClickFinger2" type="string">3</merge>
        <merge key="input.x11_options.ClickFinger3" type="string">2</merge>
        <merge key="input.x11_options.HorizEdgeScroll" type="string">0</merge>
        <merge key="input.x11_options.VertEdgeScroll" type="string">0</merge>
        <merge key="input.x11_options.VertTwoFingerScroll" type="string">1</merge>
        <merge key="input.x11_options.HorizTwoFingerScroll" type="string">1</merge>
        <merge key="input.x11_options.HorizScrollDelta" type="string">0</merge>
        <merge key="input.x11_options.VertScrollDelta" type="string">40</merge>
        <merge key="input.x11_options.PressureMotionMinZ" type="string">10</merge>
        <merge key="input.x11_options.FingerLow" type="string">16</merge>
        <merge key="input.x11_options.FingerHigh" type="string">80</merge>
        <merge key="input.x11_options.FingerPress" type="string">256</merge>
        <merge key="input.x11_options.PalmDetect" type="string">0</merge>
        <merge key="input.x11_options.PalmMinWidth" type="string">10</merge>
        <merge key="input.x11_options.PalmMinZ" type="string">200</merge>
        <merge key="input.x11_options.MinSpeed" type="string">0.8</merge>
        <merge key="input.x11_options.MaxSpeed" type="string">1.2</merge>
        <merge key="input.x11_options.AccelFactor" type="string">0.10</merge>
        <merge key="input.x11_options.MaxTapMove" type="string">25</merge>
        <merge key="input.x11_options.MaxTapTime" type="string">223</merge>
        <merge key="input.x11_options.MaxDoubleTapTime" type="string">200</merge>
        <merge key="input.x11_options.TapButton1" type="string">0</merge>
        <merge key="input.x11_options.TapButton2" type="string">0</merge>
        <merge key="input.x11_options.TapButton3" type="string">0</merge>
        <merge key="input.x11_options.RTCornerButton" type="string">0</merge>
        <merge key="input.x11_options.RBCornerButton" type="string">0</merge>
        <merge key="input.x11_options.LTCornerButton" type="string">0</merge>
        <merge key="input.x11_options.LBCornerButton" type="string">0</merge>
      </match>
    </match>
  </device>
</deviceinfo>

Reboot, and you should now be able to disable the touchpad with gsynaptics.
Note, System > Preferences > Mouse also has a section for touchpad, so disable it there as well.