I know this is possible with Linux. At my previous job one of the developers used to pipe his music stream to the broadcast address on our LAN. I don't remember much about the exact protocols involved, but I'm sure it's possible. Even better, do multicast, since broadcast is out in IPv6.
Airplay doesn't really do this. The sound is often slightly out of sync but it's enough to be really irritating if you can hear two of them. The best solution I know of for perfectly synchronized wireless sound is Sonos (quite expensive, but I love it)
