If it was carb-icing that wouldn't actually help all that much. You have two air intakes, one hot, one cold. When the engine gets warmer a flap closes and the air supply changes from the hot, to the cold. Therefore if they're both hot, carb-icing is reduced significantly. Problem is though, hot air takes up less space than cold air, so you have a fuel rich mixture, resulting in less mpg. Thats the reason the flap is there, to have cold air. In the summer its fine, but in winter often the air is too cold, cooling down the fuel (hence carb ICING), so you have an air rich mixture, which often results in stalling.
Does this all make sense? Hope it helps.
On some cars the flap isn't controlled by the engine heat, but by the driver manually under the bonnet, by summer/winter settings.
