user35553 is right, setting the FingerHigh and FingerLow values via synclient should produce the effect you're after.
synclient is a convenient way to play around with the various options the Synaptics driver offers, but if you’re happy with your changes, you’ll probably want to make them permanent for all users rather than running synclient manually or at login.
Normally you'd do that via the xorg.conf file, but Ubuntu, along with several other distros, has effectively deprecated the use of xorg.conf in favour of device-specific scripts inside /usr/share/X11/xorg.conf.d. You may or may not have this directory, and if you do, you may or may not already have a file inside it called 50-synaptics.conf. Create the directory if necessary, then open the file (substituting gedit for your editor of choice):
sudo mkdir /usr/share/X11/xorg.conf.d
sudo gedit /usr/share/X11/xorg.conf.d/50-synaptics.conf
Edit the file to include Option lines that set your desired FingerHigh and FingerLow values. Mine looks like this:
Section "InputClass"
Identifier "touchpad catchall"
Driver "synaptics"
MatchIsTouchpad "on"
MatchDevicePath "/dev/input/event*"
Option "FingerLow" "10"
Option "FingerHigh" "16"
EndSection
Save the file, and when you restart, your new settings will apply everywhere in X.
Best Answer
Ubuntu 11.10 doesn't support proper detection of trackpads for macbook pros (I'm assuming it's an integrated button trackpad). This is changing in 12.04. However, for the time being I would recommend using this driver:
https://github.com/BlueDragonX/xf86-input-mtrack
(Thanks to C S for pointing out it's in the repos) Install the driver: xserver-xorg-input-mtrack
Add this to your xorg.conf file after installing the driver:
Restart your xserver (or computer) when finished. There are other fun options, take a look at the github page for more.