If you frequent the library (or quiet places) then Breakaway could be a god-send. You'll never be afraid of your headphones becoming unplugged when you have John Mayer playing loudly in iTunes. If you have ever used an iPod, then you know that when you unplug your headphones while playing a song it is automatically paused; Well, Breakaway does this for your Mac and iTunes.
You can choose from many different settings in Breakaway, including setting triggers and fade in/out speeds. The only downside to this application is that it doesn't work with USB headphones, only ones plugged into the audio out port on your Mac. However, this freeware is handy to have around.
You can download Breakaway for free from the developer's website.