I had a request to provide information on how I implemented an ESC fail safe on my quadcopter and posted a small image of the process under the ArduCopter 2.5 firmware thread. Since I suspect there may be a more general interest in this, I'm posting a blog segment on this fail safe.
Above is an image of the process.
Also, here's a pdf copy
Edit: the Pololu devices are Pololu Servo Multiplexer, Part # 721. pololuservomultiplexer