Presumably, yes. Think about it, when you idle, you getting 0 mpg. Thesedays, it doesnt take that much fuel to start a car. I've read in many places (including BMW Magazine) that 20-30 secs is when its worth it to shut off the engine.

I don't stop the engine for such short periods, but there are a couple intersections in my area, where the extended signal cycles allow me to shut off the ignition. I timed it before; the red light on one of them was almost 3 minutes!!!