Teardowns I've Worked On
You can get a GPS fix in flight mode. The chances are that you're trying to get a location inside a plane which will block the GPS fix (as planes act like Faraday cages. Nothing in the iPhone's software actually prevents you getting a fix. The chances are, if you get a GPS lock outside the aircraft, then keep it next to the window while in filght, that it will maintain a GPS lock OK (getting a lock is harder for a GPS than maintaining one it already has). Therefore Alan's answer is not correct.
My amend suggesting the purpose of the rear mic was deleted by Brett, but the rear mic is *definitely* used for recording sound when filming video. To verify, blow on the mic during recording, then temporarily cover it with your finger for comparison. Covering this mic with your finger totally mutes the wind noise.