Just buying a cheap dock might be a better idea, but it you want to get really deep into into it....
A design class at Cornell designed something like an Ipod dock from scratch.http://instruct1.cit.cornell.edu/cou...l45/index.html
They found the pin out for the Ipod connector and the Linux Ipod people had already hacked what Apple calls the Apple Accessory Protocol, which basically tells what 0's and 1's you need to send the Ipod to command it.
As for the source for a speaker amp, I'd ask the speaker guys about that, but I'd imagine that you'd just bias the coil in the speaker with a DC voltage and then the music signal from the Ipod would just modulate it.
Also, if you're going to hack an Ipod cable, you're still going to need to have the right voltage to send across the cable. Ipod charges on 3.3V so unless you can find a transformer that steps the wall voltage down to 3.3V, you'll need to set up a little voltage divider. And if you're going to be using a microcontroller to command the Ipod, then the microcontroller will most likely take commands at 5V, so you'll need an amplifier to take the 3.3V Ipod commands to a 5V command that the microcontroller can use.