Developer and hardware hacker James Laird reverse engineered Apple's AirPort Express and packaged the important bits into a Perl script that turns your computer into an AirPlay destination.
Note: This is pretty geeky stuff, and you can bet that in a short time, someone will have cobbled together a more user-friendly version. The current script requires a lot of setup, and isn't for people unfamiliar or uncomfortable with command-line work.
Here's how to set it up, straight from Laird's README:
1. Make sure avahi-daemon is running and the prerequisites are installed. (Ed. note: Prereqs include openssl, libao, avahi [avahi-daemon running and avahi-publish-service on path] , and Perl, along with the following Perl modules: HTTP::Message, Crypt::OpenSSL::RSA, IO::Socket::INET6.)
2. Edit shairport.pl with your favourite text editor to set the access point name and/or password, if desired.
3. `perl shairport.pl`
The triangle-in-rectangle Airtunes logo will appear in the iTunes status bar of any machine on the network, or on iPod play controls screen. Choose your access point name to start streaming to the Shairport instance.
In short: This hack really isn't for the faint of heart. Whether or not you've got the chops to set this up (I'm currently testing it on my Hackintosh), the main accomplishment is the RSA private key that Laird extracted from the AirPort Express. This should allow developers to make much friendlier tools for turning any of your computers into an AirPlay-friendly device. (XBMC plug-in, anyone?)
We'll keep our eyes open, and let us know if you stumble onto a friendlier version.