I have a windows pc running airfoil, and iphone running Airview, and an apple tv running XBMC. My A/V amp doesn't directly connect to the network, but it is hooked up to the
AppleTV box via HDMI. AppleTV recieves AirPlay Audio/Video either from PC or iPhone and then sends it onto A/V amp. I am pretty sure AirPlay is multicast, in that it could simultaneously send it's signal to multiple AirPlay devices (indeed i think there is a market in speakers that have airplay built in for just this reason.)
You could probably get all this done using streaming from VLC, which i notice you are exploring in the original thread, but i have yet to find a codec that works quicker than AirPlay, but YMMV.