There is a very easy way to get that preference pane to show up and work. Simply remove the line that says VendorID (or the entire block that says NSPrefPaneIOServiceAttributesToMatch) in the Info.plist in /System/Library/PreferencePanes/Trackpad.prefPane/Contents. An easy way to do it is the following:
1. In terminal backup the original pane:
sudo cp -R /System/Library/PreferencePanes/Trackpad.prefPane /System/Library/PreferencePanes/Trackpad.prefPane.orig
2. If you have Developer tools run Property List Editor as root:
sudo /Developer/Applications/Utilities/Property\ List\ Editor.app/Contents/MacOS/Property\ List\ Editor
(if you want you can run TextEdit as root similarly)
3. Open (from the Propertly List Editor) /System/Library/PreferencePanes/Trackpad.prefPane/Contents/Info.plist and under the only expandible tab, delete the key value pair for VendorID (delete the key and value):
or if that doesn't work, delete all of NSPrefPaneIOServiceAttributesToMatch. Save the file.
4. Clear our prefPane caches (or everything) in /Users/<Current User>/Library/Caches/
5. Reboot (you might have to do multiple reboots).
Hope this works and is helpful. Tell me if there are problems, and please post the type or trackpad you have and whether it is PS2 or not.
In System Preferences you should now see a Trackpad Preference Pane with all the old options.
If you want a faster way to get this to work and have a PS2 trackpad try this (thank you SilveRain):
(Note: Use this only if your PS2 kext's are loaded from /S/L/E)
If this does not work then another option is to replace the Keyboard.prefPane in 10.5.6 with the one from 10.5.5 here: