Is there any information out there on the quality of various iPod Touch and iPhone models audio-in via headphone jack? Are there objective measures of the quality of the preamp and ADC?
In the line of iPods until model "iPod Touch 2G" Apple used analog line-in (specific pins in 30-pin apple dock connector). There was even some prosumer products like Alesis ProTrack that allowed to connect various sources to that input. It's legacy now since the Apple choose to replace the analog input with digital connections.
For modern iPhones/iPods, there two options to connect external audio sources: