Audio-in and audio-out on one connector
It has been discussed on the list that it might be useful to have connector for line-in. So far I have found that the price would be too high, as the device will then have to have yet another connector. According to [http://ipodlinux.org/Recording iPodLinux] it is possible on some iPods to use the same connector for both audio input and audio output.
In theory: When in audio-out mode the line will be controlled by a ADC and when in audio-in mode it will be controlled by a DAC. It would rarely make sense to have both the ADC and the DAC be active at the same time.