I remember hearing/reading many years ago, that if you hold a watch horizontally and point the hour hand (non-digital-display, obviously) at the sun, then the direction of Noon on the clock is where North is. As the sun gets later and moves further round, so does the hour hand, hence noon always points at the same place. Although I'm not sure if its very reliable at extreme setting/rising angles of the sun during certain parts of the year.

Could you use one of the cameras to detect the sun direction (if visible and if the phone is flat) and do the same thing, as it knows the time and the season (as it knows which country you're in)? Doesn't work at night, obviously (I don't think using the stars is a feasible idea)... Or when inside, or its very cloudy.