While not about the iPhone directly, I would still suggest checking out the Kobo e-reader option. The display is black and white but it is always readable in flight, no matter what the sun direction is. You can put the "system" together for under $100. Kobo Mini's are out of production but still around for $60, A GPS chip is around $30, and the wiring is up to you or your local computer repair shop. Runs XCSoar or Top Hat.

For me readability in flight is a top priority, and you can always read a book on it.
http://www.50k-or-bust.com/Kobo%20XC...Notes%2001.pdf

Gary