Just so everyone understands...self-defense is an affirmative defense, in Zimmerman's case it's to the murder charge. Basically, how it works is that the state must still prove the homicide beyond a reasonable doubt. The burden of proof then shifts to the defense to prove by a preponderance of the evidence that the homicide was done in self-defense. If the jury finds the homicide was committed in self-defense, basically the rule it was justified, and acquit the defendant.
In reading over some of my last posts, there is one thing I didn't make clear. Zimmerman's defense, and any self-defense argument, basically concedes the homicide occurred, though they believe it was justified (done in self-defense). The defense does not concede the murder. Big difference in those terms. By conceding the homicide, they are saying "Yes, our guy killed that guy, but it was justified." They do not concede the "murder," as that term by its very definition means an unwarranted taking of life, usually with some type of malice.
What dogmush pointed out above is that Florida has passed a law which says that, in your own home, and against an intruder (someone who is unlawfully in your home), any homicide is presumed to be done in self-defense. The state must prove that it was not justified, which would be something very unusual. The "stand your ground" law that everyone wants to bring up in the Zimmerman case doesn't change the burdens of proof, or that Zimmerman must prove he acted in self-defense. It removes from the law of self-defense the burden of proving that you had no way of retreating from the danger. Some jurisdictions require that a person "retreat to the wall" before using deadly force against an attacker. In other words, you couldn't shoot back if there was any way to run from your attacker. (Seems kind of dumb, huh?) This is why I've believed from the beginning that the Zimmerman case has nothing to do with the "stand your ground" law. What it has to do with is whether or not Zimmerman can convince the jury that his shot was fired in self-defense. Did he reasonably believe that he needed to fire to protect himself from grievous bodily injury or death? (Some states have also included reasonable belief that a person was going to be raped in this definition of self-defense.)
That all said, I think that this case has now become a circus of the law. The state and defense are spending so much time now arguing about crap that is completely irrelevant to the facts of the shooting, and putting on a good show for the cameras, that any outcome is going to be problematic for us as armed citizens. An acquittal may mean that a jury may expect us to put on evidence that our attacker was attempting to buy a gun, and bragging about other fights, and had THC in his system. A conviction may mean that shooting may not be deemed a reasonable response to an "unarmed" beating, as I heard some idiot talking head say on CNN last night. "You don't bring a gun to a fist fight" is just a stupid ting to say. IMHO, a smart man brings a gun to any fight.
Rant off.