I'm using AirPlay to stream Google Music from either my computer (using Airfoil) or from my phone (using AirAudio). The main reason for me setting it up like this is that I plan to have 2 SD cards, one for home and one for out and about where it acts like a wireless hotspot. By using AirPlay I can have others connect to it and play music from their phones.
Makes sense, thanks
. Think I'll do something similar. Are you running Airplay in XBMC? Or is there another OS you'd recommend?

Best of luck setting it up