The title sums it up pretty well. It won't play radio, shared streams, ITMS music (or the previews in the store), or even anything in my library, be it AAC or MP3. I was listening to somebody's shared music (SeverStore ROCKS!), when a song started rebuffering pretty constantly. Eventually I hit stop (in the rebuffering dialog), and since then, nothing will play, even after I restarted iTunes. The thing that shows how far the song is doesn't move, although I can drag it. Still nothing plays, and yes, the volume is up. Running iTunes 4 on iBook 800MHz, 640MB RAM.