Auto mute macbook on headphone removal
Unplug/disconnect your headphones, and mute the system volume. For added security, turn the volume all the way down. Plug in your headphones, and your volume should remain the same. The system will automatically switch between mute/no volume and the volume you have set for your headphones.

This is something that works on at least OSX 10.8 (I can't speak for sure on older versions of OSX), and a late 2008 or later Macbook. I tested it on both bluetooth and corded headphones.