If you mean to learn to play piano by using ONLY an iPad (and no external MIDI keyboard/digital piano), NO App will ever do the trick, IMO.
If you have a MIDI keyboard, and connect to the iPad, there are MANY apps to PLAY piano (ie - iGrand, etc.)
There also are many learning apps available which I am not familiar with.
I suggest that, if you REALLY want to learn piano while using an iPad, find an online course that will allow you to display their course material on an iPad, and learn on a REAL piano or keyboard. One such course can be found at pianoforall.com, but there are others.