What devices can receive an iOS AirPlay audio stream?
Yes, this would be known. 

First, the old Apple TV won't be getting the update (since its now iOS based), and therefore wont support airplay. 

Not sure of any workarounds for an old Apple TV that use the actual AirPlay standard. A hack may exist after 4.2 has been out for a while, but I think most people decided to just move to the new Apple TV instead.