Tell HN: Touch typing on a touch screen - piano?

Yesterday a friend showed me his iPad. He said something like "You can't touch type, of course." That seemed wrong at the moment, but I didn't want to argue.

This morning I awoke with a thought: Assuming the device can't provide tactile feedback, what about auditory? What if the device acted like a piano?

Feel free to expand/implement this idea and/or tell me it's been tried before and why it won't work.