I bought a splitter ($7.65 on Amazon)to access the headphone/mic input on my HTC Thunderbolt.
An Audio Technica ATR6550 shotgun mike works fine with the built-in camcorder software. Note that the impedence on this is 2200 ohms.
I have half a dozen other microphones, and NONE of them work (some have an impedence of around 1000 ohms, so maybe this is the problem).
So, here is what I know:
1. This is possible (at least with the atr6550).
2. The necessary splitter (i.e. splits input jack into mic + earphone connectors) is not expensive, and there seems no reason to use any different connector.
3. I have half a dozen other mics and NONE of them works.
4. I don't know if the incompatibility of the others is an impedence problem or a voltage / current problem, and don't know how to find out.
Here is the splitter info:
StarTech.com MUYHSMFF 3.5mm 4-Pin to 2x 3-Pin 3.5mm Headset Splitter Adapter - M/F
Sold by: Amazon.com LLC