Kawai Gakufu Camera
I just tried the Gakufu camera app for iPhone released by Kawai.
I have downloaded it with my Japanese itunes account and it is really useful for me.
You can take a photo of a line of music and it will automatically identify the notes. You can then make corrections, select the key, choose whether it is in treble or bass clef, store the recognized music and even transpose it on playback.
To play the notes, you can either swipe across the notes or you can manually tap the rhythm on the play button.
I use it as a tool to help me train my rhythm for songs that i’m having trouble with. So I simplify things by taking out the difficulty of finding the right keys to play and focus on getting the rhythm right. Then when I have that figured out, I find I can play it better when I go back to the piano.