Hopefully this post will clear up some of the discussion around this topic.  I recently did an interview with Susan Bartelstone of Crime Prevention 101 in which we discussed this issue and where the concept came from.

Here are a couple of the key points from our discussion.  One yelling fire can improve your chances of someone coming to the scene.  Second, it is meant to be your internal call, your survival call, your chi, your kiai, your get away instinct, your inner fire, what ever label you’d like to give it.  It is you making that call for the strength to preserve your life.  Yelling fire is not a replacement for any self-defense technique, it is used to create attention and to break you out of freezing up and turning on your strength to survive. 

We find that yelling fire can be helpful, but do what feels right to you.  Most importantly, get attention and attack the attacker.