Developer James Laird decided to reverse engineer the Apple Airport Express to find the private key after his girlfriend had connectivity issues and he was surprised that an Airport Express emulator didn't already exist. With the key in hand, he then wrote his own open source AirPort Express emulator called ShairPort. Here's how he describes it:
This program emulates an Airport Express for the purpose of streaming music from iTunes and compatible iPods. It implements a server for the Apple RAOP protocol.
Previously, when the private key was still unknown, you could use iTunes to stream music to Airport Express or use official third party solutions to stream music to Airport Express. Now, users can use iTunes to stream music to third party software and hardware. By emulating Airport Express, you can trick iTunes into thinking it is streaming to AirPort Express, when it actually is streaming to anything you please, which puts quite a bit more options on the table.
Of course, the hardware part isn't too big of a deal: hardware manufacturers likely won't use unauthorized information to create AirPlay-compatible hardware products, given that they can simply avoid Apple's wrath by licensing to become an officially AirPlay partner. That being said, unofficial firmware for popular hardware could definitely get a boost. On the software side, this means iTunes music could be streamed to other Macs, PCs, consoles, or mobile devices with the right applications.