To get to your question, Jim, I can speak to Florida's laws on this. I've had to use a firearm in self-defense twice in my life, and it was shortly after I turned 21. I was in college in Tallahassee, and this happened even before Stand Your Ground was law.
It removes the duty to retreat or de-escalate. It also allows any lawful gun owner to have their weapon with them in any place where they are legally allowed to be. So, for instance, they can have a pistol on their side in their home, on their property, in their vehicle, boat, etc. They can also have a firearm on them in a friend's home or vehicle, provided that owner is okay with it. Since you are required to carry concealed and Florida does not allow open carry (yet), you can't legally carry a firearm in a restaurant, grocery store, WalMart, etc. despite being legally permitted to be there.
Zimmerman was not on duty for the watch, but he was on a personal errand. From his home to his vehicle and on the road, he's legally allowed to have that firearm with him. As soon as he got out of that vehicle, he can't legally have his gun unless he has a CWP (did they mention yet if he's a holder?). This is where the story gets a little muddy. He went looking for Martin, but broke off the search. At some point back to his vehicle, he was attacked. At this point, Stand Your Ground permits him to defend himself regardless of the disposition of force. Florida law makes no accessions for equality of force.
Someone comes at you with a weapon, you only have to be in fear for your life to shoot. That's the key phrase, even I had to use it in my situation: "I was in fear for my life." Period, case over. Law enforcement cannot, under Florida law, arrest you for pulling the trigger. Maybe they come knocking on your door at a later date to follow up, but you walk away.
Thanks.