The behavior of an iPod on your Mac!
The iPod automatically pauses whatever is playing when you unplug the headphones. It is possible to get the exact same behavior on your Mac with an application called "Breakaway". Breakaway runs in the background with an icon in the menu bar. By default, as soon as you unplug the headphones, it pauses whatever is playing. You can also set it to pause when you press the mute button. It is also possible to temporarily disable it without actually needing to quit it. You can learn more about Breakaway or download it here from MacUpdate.