I've often wondered why this isn't possible myself. I have a Mac Mini in the study and an Apple TV in the living room and occasionally I want to play music from speakers in both rooms. Initially I did this by playing from iTunes on the Mini, which works fine, but required running back and forth to change music. Then I found Remote which works pretty good, but.... used Airport Express's are relatively cheap these days, so I've disconnected the speakers from the Mac Mini and attached them to an Airport Express. Now I can Airplay to both rooms from The Mac Mini or one of our MacBooks. To get system sounds to play over the speakers on the Express click on the speaker icon while holding down option (only works in ML) and you can select the express from there. I believe Airparrot was the idea before ML.
Ideally, I would like to be able to play directly from my iPhone to multiple locations. Couldn't a program running on the Mini manage this, which I believe was the OP's question. I suppose a software implementation of a receiving device might exposes too many secrets of the Airplay technology??