I was listen to spotify (music application) on my Macbook running OSX Leopard and found that when pressing the play/pause button on my iphone headset cord that I had connected to the laptop, it automatically stopped and started the music. Jumping tracks would not work though. Volume worked fine but that was expected as it controls the entire system's volume. What was quite interesting is that Spotify was not that active/frontmost app but was playing in the background. Is OSX able to intelligently detect what application supports play/pause and is running at the moment? What happens when more than one music or similar app is running? Spotify's mac app does not seem to have preferences for controlling any such functionality.