To disable the touchpad:
Click
Uncheck
If the touchpad is your only pointing device and you accidentally turned it off, you can turn it back on again using just the keyboard.
Press
This section needs fixing-up to provide useful advice. It's basically working around a feature request bug.
There is currently no simple way of having your touchpad automatically disabled when a mouse is plugged in.
gconftool --set /desktop/gnome/peripherals/mouse/touchpad_enabled -t bool True
gconftool --set /desktop/gnome/peripherals/mouse/touchpad_enabled -t bool False
ENV{ID_CLASS}="mouse", ACTION=="add", RUN+="synclient touchpadoff=1"
ENV{ID_CLASS}="mouse", ACTION=="remove", RUN+="synclient touchpadoff=0"