Use this example as it shows how to turn pins on and off over the internet.
http://bildr.org/2011/06/arduino-ethernet-pin-control/
I used this as the basis for a controller to turn lights and pumps on and off on my fishtank. I can do it from anywhere in the world as long as I have an internet connection. You will need to set up the arduino/ethernet shield combo as a webserver at the location where you will have the red "you screwed up" light. Connect it to a router and enable port fowarding on the router to route all port(??) requests to the ip address of the ethernet shield. I had a problem using Port(80) when I did mine because my ISP blocked it. I used port(150) and it worked fine. Just send commands over any web browser.