What devices can receive an iOS AirPlay audio stream? In addition to printing, the other feature in iOS 4.2 that I'd been looking forward to was AirPlay. My understanding from the keynote when it was introduced was that it allowed iOS to stream audio to existing AirTunes equipment (as well as provide video to the new Apple TV), but I can't get my iPhone 4 running iOS 4.2 to stream audio to my old Apple TV like iTunes can. Is this a known problem? Are there any workarounds, besides buying one of the new Apple TVs (which I can't because my TV is component, not HDMI)?
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.