Analog wired Headset
From Openmoko
There's a four-ring 2.5mm stereo jack which provides connectivity to old-fashioned wired headsets.
The headsets used by Motorola smartphones (A780,A1200, ...) and the V-360 have a compatible configuration.
The headsets used by some Nokia phones (N95, ...) have the same jack but they are incompatible. For technical information about rewiring them visit Analog wired Headset Nokia
Neo FreeRunner
ring
1(base): GND
2: left out
3: right out
4(tip): mic + HOLD-button(press:short to GND)
Neo 1973
Pinout: [1]
base = ground speaker left (internal impedance 33R) to ground. (+jackinsert detection) speaker right (internal impedance 33R) to ground. tip = mic electret condenser type, to ground. bias (power for mic) 2K2 from +3.3v(wolfson codec) (+HoldButton shortcircuit to ground)
Software support
Dylan proposed a proof of concept scripts to fix something about the headset insertion/removal being incorrectly acted upon.