For a long time now, apps can stream high-quality audio to an Airport Express or an Apple TV using the RAOP protocol. However, the reverse cannot be done due to the fact that the protocol uses asymmetric encryption, which means the private key is baked into the firmware of the Apple (or Apple-licensed) device.
Finally, someone has done something about it. James Laird dumped the ROM of his Airport Express and extracted the private key. He posted the private key to the vlc-devel mailing list.
And now, the site on which he hosts his implementation called shairport is returning HTTP 500.
Update 13-Apr-2011: The link to shairport and his site is back up.