How far is your drive to/from work? It's possible that you're not driving the car far enough to allow the alternator to fully recharge the battery after the drain for cranking the engine. I have seen this in the motorcycle word many times so I can only assume it works the same way with cars.

I average about 150-200 highway miles a day on my Crossfire and the lights do not dim at idle. I will see a slight dip in brightness when the cooling fan kicks on but that only lasts for a second or so.

Bob.