A cell phone jack has to accommodate headphones that have a mic in them. If you compare your regular ipod headphones to the iphone head phones you will see the iphone has 3 stripes on it and the ipod has 2 stripes. The extra section is to accomodate the microphone in your headset.

Usually the adapter drops one line internally for a mike in short adapter section and then passes on the other two lines for your standard headphones.

This is pretty much a know deal, i had a I730 and it was the same thing. Your iphone speakers probably will not work with your ipod.