It's not going to teach you how to play piano but it might teach you about timing and coordinating the fingers. The app is called Magic Piano by smule. It's similar to any Rock game app where you have to press the button at the right time in the right spot.
Each dot will indicate a musical note. Songs like "Twinkle Twinkle Little Star" and the theme to Mario Brothers are free songs you can play. If you are off the mark the game will let you know. If you are hitting the dots too slow or fast you will be told to speed up or slow down.
If you are on mark your score will increase. If you miss the dot the song will continue until you can get back on track and increase your score.
You can purchase songs in-app starting at $2.99. Magic Piano is free and comes with a few songs to try. It's available for all iOS devices.
Check out the previous iPad app – Money for iPad