(Only 3rd and 4th Generation iPods have this, it is a the small oval port next to the headphone jack).
It's been quite some time since iPods have used this iPod remote connector, but the dock connector (the one used on all iPods except the shuffle, iPad, and iPhone) has the same Rx/Tx pins, as well as a 3.3V out. A simple breakout board can replace the hacked connector at the end, and you can get this to work with any recent Apple products. You can buy breakout boards at: http://www.kineteka.com/PodBreakout-v1.aspx
(The mini one is pretty nice, and they also have pinout information).
Step 1: Parts
Programmer- the drawback to using a dsPIC is the complicated programming procedure. The easiest way to program it is to use a Microchip ICD2, however these run fairly expensive. I have not tried this, but apparently the utilities found at http://homerreid.ath.cx/misc/dspicprg/ can be used with a homemade JDM Programmer.
IC sockets- I used 2 8-pin DIP sockets(a single 18 pin or 16 pin would have also worked). These are necessary for removing and replacing the IC for programming and debugging.
Dremel with a cutting bit
Soldering iron and small gauge electrical solder
Flush cutters, or wire cutters
Standard Ethernet (CAT-5) cabling
A good amount of small gauge wire- I used the innards of extra CAT-5 cable.
3G or 4G iPod.
A plug for the remote jack on the iPod. This is the most difficult to acquire. Several suggestions are made at iPod Linux's site.
I used a small piece of a shattered old memory module that perfectly fit the remote plug, but any of the other solutions also work.