Electronic aspects of iPhone 3.5mm audio output
This is the data I can share with you after having run my own set of experiments and having search (extensively) through the Web for other people's real hands-on tests. I have discarded / omitted the data which I have been unable to reproduce:


The impedance of the standard Apple miniature hands-free microphone, the one integrated with the headphones they include in the iPhone 4th gen device is about 1600 Ohms. This means, if you simply connect a 1.6K resistor between the MIC and GND connections of the TRRS connector, the iOS will switch to the external microphone (in fact, a resistor, not a microphone).

Having said that, the truth is that different versions of the iOS and devices will employ different decision algorithms when trying to "guess" if there is an external microphone connected to the iPhone, iPad or iPod. You can find some references in the Web (I won't cite them here as I consider the information misleading) stating different threshold impedances and behaviours for the algorithms of each iOS version and device.

My advice is simple: forget about the gory details of the different versions of iOS out there. Simply use a 1.6K resistor, mimicking the genuine iPhone miniature microphone. I'd bet Apple is not going to change the iOS behaviour in the near future ruling out millions of hands-free headphones!
The iPhone as well as other mobiles will apply a DC voltage about 1.5-2.5V to the microphone. The objective of this is twofold: it serves the iPhone as a way to measure the external DC impedance of the microphone and also powers the preamplifier built into many miniature electret microphones.

The above means you should be careful when interfacing to the mic connection of an Apple device, either by AC coupling your signal on top of the 1.6K previously discussed resistor or by using a voltage divider with a bigger resistor and again the said 1.6K resistor. Which leads to the next topic...
The microphone input of the iPhone will saturate at about 40 mV peak (millivolts). Thus, you must adapt the amplitude of your audio signal to similar level.
Beware of the audio 3.5mm TRRS plug (male) connectors out there! Before using one, look carefully at the connection at the base of the plug (the S or "sleeve"). Some connectors will end in a circular metallic disc connected to the sleeve.

The problem is, many Apple devices (for instante, the iPhone 4) have a metallic enclosure internally connected to ground and this type of connectors will make contact between the enclosure (ground) and the sleeve (microphone), rendering useless all your tries to inject an audio signal. I have suffered myself this problem in the past, wasting some hours trying to figure out why the iPhone won't recognize a properly connected 1.6K resistor.

You can find some nice pictures of this problem and a temporal homemade solution for it:
http://martinjohnsoncommunications.blogspot.com.es/2012/04/iphone-external-mic-connection-solved.html