Interface netduino with Iphone via audio jack
There is a project developed by David Jurick, Adam Stolarz, Damien Stolarz that is described in the book iPhone Hacks. They use the audio jack as a Frequency Shift Keying (FSK) modem that operates at about 1200 baud. Their project interfaces with a Cypress PSoC microcontroller to convert the audio signal to RS232. Source Code can be found at the www.perceptdev.com site.

A similar project using an Arduino to convert the signal can be found here.

Another project called HiJack harvests power and bandwidth from the iPhone's audio jack.