Just a theory, this, as I don't know the ins and outs of today's speaker technology. But...

I imagine it would be more difficult to effectively replicate the output of a piano than the input. The input can be simply put together digitally using sensors, eventually if not using existing technology. The output will be a little more difficult. Having said that, I'm sure it could be done. The hurdle is essentially the size and shape of, say, a grand piano. The sound that we hear, where does it come from? A felt covered hammer striking steel strings, apparently. Can that be effectively replicated using today's typical speaker systems? I know you can get the basics of it, just like when you play an mp3, but if you are standing in a room with a grand piano, the sound moves around a lot more. It's not just the impact point that causes the sound, to my understanding - it's the entire string. Would sound move out of a speaker in the same way it moves off a string? From what little I know, I don't believe there are speakers which allow for this kind of sound source in shape and size, meaning it would be difficult to replicate effectively.

EDIT: Oh, I just found out that there is a sounding board, not just a string. Never mind then. Still, I'm interested in knowing how easy it would be to replicate effectively the way the sound originates and moves throughout the piano and, subsequently, the room around it.