Well i dont have numbers to tell for sure but as far as wear is concerned, it is more at startup. And at idle, car can have 2-3 times more mileage so i think isbcap is right.
But there is a new theory + technology ie start-stop. According to them, engine at running temperature (such as when we stop at signals) consumes far lesser fuel at startup so the ecu basically shuts down the engine. As soon as pedal is pressed, the engine is restarted seamlessly as if it never stopped and hence they say this technique saves fuel. Still its a debatable topic personally i keep the engine on (search for my posts, my civic gives 18+ km/kg)
And i remember last year summers i idled the car for an hour with ac and it hardly made a difference on mileage.
For more info, google startstop