I had written about this small applet that shows touchpad status and provides quickly enable and disable touchpad functionality in Ubuntu laptop and netbook.
Now,the Touchpad-Indicator has updated with keyboard shortcut and disable touchpad when mouse plugged supported.

Installation or Upgrade

Get the latest version and keep updating by adding developer’s ppa, run following in a command console (Applications -> Accessories -> Terminal)

sudo add-apt-repository ppa:atareao/atareao
sudo apt-get update
sudo apt-get install touchpad-indicator

Touchpad-Indicator can be found under Applications -> Accessories menu.

New feature:

First start touhcpad-indicator and enable this feature in its Preferences window:

Click configure, and do follow the popup dialogs: