I'm having the same problem on my 2007 as well. I did notice this:

I have my headlights set to Auto. So when I start in my garage in the morning, due to dim lighting, the headlights are on when the car starts. After the car sitting all night in the garage, the screen brightness has dimmed to half way (per the settings screen).

I change the screen brightness setting to max and it stays that way throughout the day, even when it gets dark, the screen is still OK.

But when the car sits overnight again, the same thing happens--so it seems to be based on the amount of time the car isn't driven (maybe when certain system finally timeout, the setting isn't getting saved?).