Shorting two pins using a headphone jack
Neat idea. I just finished building a Bluetooth controlled Canon SLR remote, but it's controlled from the phone, not by the phone. 

I think you can use this idea to drive the input to a MOSFET (a 2N7000 should do). When the signal from your phone drives the opamp high, the MOSFET will act like a switch, shorting your shutter release pin to the camera/circuit ground. If it turns out that the signal is too short in duration for the shutter to fire, just use a 555 in monostable operation in between your phone and the MOSFET, I'll bet the audio signal will work as the trigger input. If it doesn't, use both ideas together, have the square wave from the opamp trigger the 555 timer.