It is well documented that you cannot stream to more than one AirPlay compatible device through iOS. (In an early developer build of 4.0 the functionality was put in but was taken out in the GM).
I am aware you can stream iTunes content to multiple devices and even use the Apple Remote App to control multiple devices.
However, I am looking for a solution that allows me to play a Pandora/Spotify/Slacker/Songza app on my iOS device and allow me to stream that music to multiple devices.
The only solution I have found right now is a complex solution using Rogue Ameoba's AirPlay Speakers app on the iMac but there is about a 4 second lag and additionally, once you have captured audio, you have to manually go to the computer to cancel the connection to allow for other connections.
Does anyone have other workarounds or solutions to this challenge?