Developer James Laird has reverse engineered the Airport Express private key and published an open source AirPort Express emulator called Shareport.



From MacRumors:

Previously, the private key was unknown, which meant that only Apple's Airport Express or official 3rd party solutions could wirelessly stream music from iTunes or equivalent. Many existing solutions such as Rogue Amoeba's Airfoil have long been able to stream music to AirPort Express or other AirPlay devices, but not the other way around.

  Apple AirPlay Private Key Exposed, Opening Door to AirPort Express Emulators