If you are looking away to add an AirPlay receiver to your home audio setup without shelling out $100 for an Apple TV or AirPort express then you might want to take a look at what Jordan Burgess has done with a $25 Raspberry Pi.
The Cambridge engineering student successfully converted a Raspberry Pi into an AirPlay receiver using open source software and a USB WiFi adapter. The total cost was about $48, but that also included an SD card (for the Pi’s operating system) and a micro USB cable. Setting up the hardware and software is relatively easy, especially if you grew up in the days of DOS – if you have messed around in Command or Terminal, then it should be a breeze (and the Internet has a wealth of information on this sort of thing.
Personally, I think it is a great idea, but there is one pretty serious drawback: the Raspberry Pi does not do digital to analog very well, so outputting sound straight from the 3.5mm port on the Pi will result in quite a bit of noise. A USB sound card is one possible fix, but that pushes the price even closer to Apple TV / AirPort Express territory.