if it's not already installed update your Synaptics Touchpad driver to A16 as the enhancement includes:
Adds the feature where the touchpad/stick is disabled when a USB mouse is plugged in. This feature is disabled by default but can be enabled via a checkbox on the Devices tab in the Touchpad. When a USB mouse is plugged in a pop up message informs the user that a USB mouse has been detected and that the touchpad/stick has been disabled. A red ''X'' appears over the touchpad icon in the systray when it is disabled.