This is normal for most DSMs. The alternator output drops slightly when the engine is idling, and the headlights will dim slightly.
Similarily, turning on the brake lights or turn signal puts an additional electrical load on the alternator. As a result, the alternator output voltage drops slightly, resulting in slightly dimmer lights. This holds true even for Canadians-spec DSMs equipped with the larger 90A alternator, and especially at idle. It is not a sign of malfunction.