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.