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.
ring
1(base): GND
2: right out
3: left out
4(tip): mic + HOLD-button(press:short to GND)