To add to the previously published results, we decided to measure the frequency response of the iPod Touch 3G. Electroacoustics Toolbox 2.1.7 was used with a MOTU UltraLite mk3 audio interface to measure the frequency response. These measurements include the response of the headset output as well as the headset input. The audio was routed through the iPod Touch using SignalScope Pro, and the measurements were made on a 3rd generation 32 GB iPod Touch.
With each new iteration of the iPhone, the low-end roll off of the headset input has been steeper and steeper. The iPod Touch seems to be following a similar trend. The iPod Touch 2G had a 3 dB frequency around 30 Hz while the iPod Touch 3G has a 3 dB frequency around 150 Hz. If you plan on using the headset input for any type of low frequency measurements, you should definitely take this into consideration. Hopefully this trend will not continue with future iterations of the iPhone/iPod Touch.
As with the previous measurements, the dock connector input was very flat, and has not changed significantly from the older hardware. The 3 dB frequency is still below 10 Hz, with a small amount of ripple at low frequencies, and it stays flat until above 20 kHz.