A better question to ask would be, "What if George Zimmerman had open carried?" The whole point if "open" carry is that people can see the weapon and this will affect most people's behavior.
Fair enough. If Trayvon Martin had an open carry weapon, would Zimmerman have decided not to follow him? How would the knowledge that Martin also had a gun affected his behavior?
Just to keep it on topic, we haven't yet had a high profile incident where both people were carrying weapons and each stood their ground, so open carry advocates have yet to come out on that issue. Actually, we've had plenty of them, but they are are called gunfights, not stand your ground.