Which iOS devices support microphone input via the headphone jack?
Here is a summary of what iOS devices support microphone in via the headphone jack. This information was acquired by looking at Compatibility of Apple wired headset models with iPhone, iPad, and iPod models

iOS Devices that support Microphone via the headphone jack


iPod touch (2nd generation and later)
iPhone (Original and later) 
iPad (Original and later)


Note: iPod touch (2nd generation) software version 2.2 or later required to use remote and mic features.