Sorry but I'm not too sure if something like this has been posted already.
I'm currently using the Shure MPA (mobile phone adapter) to control my songs playing through my iPhone via the AUX jack. I control my songs by pressing the button on the this adapter.
What this allows me to do is the following:
Press button X: Play/Pause song
Press button XX: Plays next track
Press button XXX: Repeats track or if at the beginning of a track it plays the previous track.
Hold the button: Voice control (perhaps for making phone calls)
I haven't tested this out on an iTouch, iPod but will report back if I do.
There are other MPA's out there but the sequence in how the buttons are pressed may differ.