Why do I have to turn off my (non-cellular) iPad before takeoff?
Actually, they don't!  Well, not always anyway.

The FAA recently issued a press release titled FAA to Allow Airlines to Expand Use of Personal Electronics.  This press release allows for the use of some electronics, including iPads and similar devices, during all phases of flight,.  However, it is up to each airline to change their procedures and to determine which devices can be safely used.

This is a small excerpt from the press release:


  Passengers will eventually be able to read e-books, play games, and
  watch videos on their devices during all phases of flight, with very
  limited exceptions. Electronic items, books and magazines, must be
  held or put in the seat back pocket during the actual takeoff and
  landing roll. Cell phones should be in airplane mode or with cellular
  service disabled – i.e., no signal bars displayed—and cannot be used
  for voice communications based on FCC regulations that prohibit any
  airborne calls using cell phones.    If your air carrier provides
  Wi-Fi service during flight, you may use those services.  You can also
  continue to use short-range Bluetooth accessories, like wireless
  keyboards.


I flew on Jet Blue the day after that press release and they were already allowing iPads and similar devices as long as they were in airplane mode.  Other airlines will follow soon I'm sure, but their manuals and procedures must be updated and approved by the FAA first.