This is apparently accomplished through the use of an iPhone 4S with Siri, a song file whose MIDI data is sent to an Airport Express that has been hooked up to a Yamaha Disklavier. For those who are unfamiliar, a Disklavier is a Yamaha piano that can be operated independently without the need for a human player. If you’ve ever been to hotels or malls and see a piano whose keys and pedals seem to be moving by themselves while emitting music, that would be one such example of a Disklavier right there.
To be fair, this isn’t really so much Siri “playing” the piano, but rather Siri sending the instructions and necessary data to the piano that would allow it to play the songs that the user has requested of it, which is still pretty darn cool anyway. Check out the video below to see Siri and the Yamaha Disklavier in action.