using iTunes 10 with Airport Express broke iTunes 9 AirTunes: "An unknown error occurred (-15000)." detailed description: I used iTunes 10 on my MacBook Air 11" to stream radio via AirTunes to my Airport Express - worked fine. When I then disconnected it and tried using iTunes 9 on my white MacBook to play AirTunes, it said "Connecting ..." and eventually gave me an error alert: "An error occurred while connecting to the remote speaker 'library'. An unknown error occurred (-15000)." (Not a terribly useful error message for trouble-shooting - even a web search turns up scant clues, mentions of firewall configuration - wasn't running one). Thus it appears that using iTunes 10 with AirTunes breaks iTunes 9 compatibility on that Airport Express. Feels like another (perhaps unintentional) forced software upgrade path. Here is why this is a problem: if you visit a friend's house and play music through their Airport Express, you might then break your friend's ability to use same Airport Express to play music (they might not be able to upgrade their iTunes etc.).