How To Turn Off Arduino

Hi...

I have my new Arduino Duemilanove with me , and what I want to know is, how to stop it while it is connected to my pc using USB... What I am doing right now is, I unplug it from my PC, I change the circuit the way I want and I reconnect the USB cord. But I was searching for a switch that will let me switch off arduino. The switch that is there on my board is the reset switch and it doesn't swith the board off...

Basically there isn't one :P