I saw this story on CNN, in the video section. A 15 y/o girl who is blind, severely mentally retarded, can't add or write, but plays the piano beautifully. You'll have to see the story for yourself. I ordered her cd within minutes of watching her story.
CNN's story : http://www.cnn.com/HEALTH/ Then click on 'Prodigious savant'.
Another link with an audio demo clip and Dateline story on her.
And her official website.