The screen is dimming itself based on the percentage of white pixels on the screen (a sneaky way to improve battery life). This is not to be confused with auto-brightness which dims the screen based on the ambient light sensor.
The
Focus and Focus S did the same exact thing, except that on the Focus S, Samsung modified WP7 to include an extra settings field called auto display intensity that allowed you to disable it (Nokia did not include this feature). Nokia is using Samsung AMOLED panels so naturally this "feature" is included in the firmware on all Lumia 900 displays.
You can see it in action here:
Nokia Lumia 900 Review Part 2 - YouTube