Clever & Son released an especially clever app that will stream any audio source on your Mac to your iDevice. Things get even more interesting when you can now take that stream and plug it into your Audiobus Input slot! This is unfortunately Mac only.
WiFi2HiFi 2.0 iTunes Description:
For Mac only, Windows not supported.
Using the WiFi2HiFi application any iOS device in reach can tune in to the music you're listening to on your Mac. Cumbrous audio cables are replaced with smart streaming technology and specifically designed software for your Mac.
Plug your iPhone or iPad into a docking station, connect it to your audio system, or just use its speakers and have your Mac's entire music collection, audio streams, Spotify, Rdio or and any web radio ready to play.