I'm not sure if it is setup by default in Ubuntu, but if the Synaptics driver is set to allow SHMConfig, you can do it with synclient easily.
This turns it off:
Code:
synclient TouchpadOff=1
This turns it on:
Code:
synclient TouchpadOff=0
You shouldn't need any special privileges, so adding it to a launcher button on your panel should be trivial so you can easily toggle it. I added it to one of the special buttons on my laptop with an ACPI script.
EDIT: If you do not have SHMConfig enabled, this link should help:
https://help.ubuntu.com/community/Sy...hpad#shmconfig