Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
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.
 
I would suggest finding a decent teacher. Even if all you want to do is play pop tunes a decent teacher can put you on the right track.

I taught myself piano in my 20s. Then when I decided to go to school for a degree in piano I got my butt kicked. But after a ton of work I'm nearly finished with a masters of music. None of this would have been possible without a teacher or atleast a mentor of some sort.

As stated above though I don't think you can even start to teach yourself without a keyboard, the iPad could be used as a tool along the with a keyboard to assist your learning. I'd suggest a weighted 88 key keyboard of course this is assuming you want to be able to play piano. If you just want to tinkle around with a keyboard a cheaper solution might be ok.
 
I'm sure there are lots of piano apps out there, but I started by just using GarageBand. Granted, you don't get a full piano at your fingertips, but just taking it a few notes at a time is good enough for any beginner. You'll want to move on to an actual instrument before too long, but the iPad can get you started.
 
Like others say, it isn't as much about the apps/sounds, but the actual PLAYING the keyboard that you will want something other than a glass touch surface.

If you want to do an iPad-specific music learning, I would find something like the iRig Keys Pro (keys are non-weighted but are full sized).
 
Diffently recommend a iPad compatible keyboard. Yes, you can play without. but not the hole tangent system is on the iPad screen. For learning ? take a look at Yousician :)

Hope this helps hehe :)
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.