I saw an interesting documentary about the human voice on BBC last year. Jeremy Hardy volunteered to be a tone deaf guinea pig for a test to show that pitch perception can be improved. He sang into a mike that gave a visual read out on a large screen in such a way that he could see the discrepancy between the note he was producing and what he was aiming for. This visual feedback meant he quickly could adjust his pitch and see the effects on screen, thus quickly grasping the mechanics of vocal pitch change. It was a very thought provoking moment!