During setup the brightness is at a fixed value, but as soon as the player gets connected to SC/SN it takes over whatever brightness setting there is for a new player. If it's set at a lower level or to automatic this could result in the described behavior. In case of automatic it would depend on the surrounding light whether the display would dim or not.

Maybe SC/SN should use a fixed brightness level rather than automatic as default for new players? 

Andy to see if there's a way to ignore the sensor on a newly-connected device and then magically switch back to it.  May be a won't-fix

This is caused by the default brightness pref being to use the light sensor.  The light sensor is not used during the firmware setup menu, so when you finally connect to a server the display will likely dim.  This is normal, so I don't consider this a bug.

In that case, it sounds like the default brightness needs to be increased. The importance of users' ability to read their PIN clearly during setup cannot be understated. 

A screen going dimmer when you're about to read critical info off it is disconcerting to the user. Fortunately the same cannot be said for going brighter -- no harm there.

If the ambient sensor isn't working right, that's another bug right?

Matthew: Please open a new bug if you feel the light sensor isn't working properly.