Does anyone know how to access the iPhone's serial port, from a program on the iPod to allow that application to send signals. The idea is that the iPod could be used as the brain for some kind of robot by using the serial port to tell an arduino what to do.
Most of the suggestions need a jail-broken phone. ucHobby has a different approach using the audio jack of the phone or ipod.
This looks like what you want:
One thing to note is that you cannot use the official External Accessory Framework to communicate with accessories, unless the accessory that you're communicating with has been designed under the Made For iPod, or Works With iPhone programs
Related: Here is a tutorial on how to boot a Jailbroken iPod 2G using a microcontroller.
since OS 3.0 there is an API for the hardware port. might wanna use that,
I wrote an application GPS2IP that outputs NMEA messages (and gpx and kml tracks) without a jailbroken device.
You need to connect to your iPhone with a IP socket though, so that might be difficult for a very simple project. IP stacks for micros aren't terribly difficult to get for free, these days, though..