It sounds like your particular need does not involve MIDI or capturing notes from a digital piano, if I am reading correctly. You want just the audio sound of a piano, to play back on an iPhone app?
A few questions/thoughts:
First, do you just need one note/sound? If so you should be able to easily find a sound file that is to your liking on a site that sells sound samples. I myself use sounddogs.com for various odd projects.
Next, what sort of code do you need to play the sound? What programming language are you developing the app in and how do you trigger the sound playback? These details will be helpful in giving you some ideas.
Finally, if the main obstacle is just finding and recording the sound, this question may be better moved to superuser.com since that site deals more with general computer questions, media, and technology. (StackOverflow is focused strictly on programming.)