Mat: "Super legal" is an interesting term 8). There should be no legal issues here whatsoever.

Airfoil Speakers on the Mac and PC has had this receiving functionality for years now, and now it's on the iOS platform as well. Airfoil Speakers is a clean-room implementation of the AirPlay protocol. Further, as you note, Apple approved the app after review. So, we don't anticipate any issues here.