I have this exact same problem with my 07 LT2, here is what I have learned.
This annoys me a lot.
My dealer doesn't care.
It seems to have to do with the light hitting the sensor when you start the car. If I start the car in my garage, my headlights will remain on (with the instrument panel dimmed) for a long time. Friday I timed it was over 20 minutes of driving on the highway at 11am on a sunny day before I reached my destination. My lights were still on. When I came back and started my car again with it sitting outside in full sunlight, everything worked at it should.
I have a couple theories as to why this happens.
The sensor is borderline bad (probably the case) but I haven't bought a new one to install yet and verify.
The guy who wrote the headlight program for the car was out late the night before and still hungover when he got to work that day. All sensors like this have what is called a "debounce" built into them to prevent the lights turning on and off rapidly every time the sensor gets into a shadow. The algorithm to turn the lights ON is working properly, but the one to turn them OFF is resetting every time a shadow crosses the sensor.
Either way, since my GPS was stolen from my dash (what the dealer most recently claimed was my problem, it casts a shadow you see.....) I'm going to revisit the issue with them.
the headlights are always on but in a less than full power mode, those are your DRLs. Your sidemarkers and taillights aren't on until you turn the headlights on and/or the sensor tells the headlights to turn on for real.
I noticed the same effect on both my '05 and '08 but I just ASSumed it was because VA makes you put the inspection sticker in the middle of the windshield, unlike every other state...
one peeve, the VF display for the shift indicator and the radio are radically different brightnesses, if I set the dash lights so the shift indicator doesn't burn my retinas at night then when the headlights automatically turn on but it's still daytime I can't see the radio display anymore...
i know this is a very old post, but with cars and trucks for that matter,whenever the running and headlights get turned on, the dash lights automatically dim for night viewing but are not affected when the DRL's are on by themselves.
I had the same problem on my 03 Base model. The headlights are on vice the DRLs and the "High Beam H/L out" idiot light is on with the Brights indicator light on. Tuns out I have a short on the drivers side h/l harness......not saying this is your prob, but couldn't hurt to check your wiring.