HOUSE PARTY SUCCESS!!!
I have an ATV3 in the living room, one in the bedroom, and one in the garage (for streaming music out there too...). To get the whole compound in sync and have the same music everywhere AND control it from my iPhone, it was / is this easy:
In iTunes, select multiple airplay devices so everywhere you want the music playing is selected. Use the Apple Remote app on your iPhone / iPad and control your iTunes, NOT your ATV. Bada-bing! Full house party music to all the devices and controlled by the phone in your pocket!!!
They should still let you do this from any iOS device directly though...
Unfortunately not. Apple TV only supports the selection of ONE Airplay device currently. Seems to be a pothole in the implementation as iTunes on a Mac/PC allows the selection of multiple devices. I wanted the same solution: Apple TV without a running Mac/PC + music in the Apple cloud + multiple speakers in the house with the same music. Let's hope Apple fixes this in the next upcoming Apple TV firmware update.