I did some reviewing of the facts of the case (such as a Google Earth map of the area with the approximate locations marked). From what I can tell, here's what happened.
Martin was walking home from a convenience store with his purchases in his hands. Zimmerman was driving through the area on a personal errand and saw Martin. He proceeded to call 911. The transcript of the call is available on the shooting of Trayvon MartinWiki
wiki page, but the gist of it is that Martin saw Zimmerman, approached his car, and then ran off (away from the street). Zimmerman got out of his car when he saw Martin running and followed him, probably to see where he was going, thus the question from the dispatcher about whether he was following Martin.
I don't think Zimmerman was actually chasing Martin - just trying to observe where he went. However, considering that he went past the first row of townhouses, then down past the first one, it was more than a bit too far to simply observe him. And given the situation, it's likely that Martin (who was probably having an adrenaline rush) returned to where Zimmerman was and confronted him. That's when the fight happened. So, Zimmerman may not have actually instigated the altercation, but he did have something to do with provoking it by getting out of his car and following Martin.
Was it self-defense? I don't think it was. Zimmerman shouldn't have gotten out of his car to go after Martin to begin with, and he also shouldn't have stayed in the middle of those two rows of townhouses (especially considering his apparent worry about not knowing where Martin was). That violated two of the basic principles of self-defense, avoidance and withdrawal. By getting out of his car and following Martin partway, he upped the stakes for both of them.
Certainly, once Martin had him down and was hitting him, he was pretty limited in what he could do. But he should not have let himself get in that position to begin with. He should not have gone after Martin in the first place, and he should have pulled back as soon as he lost sight of Martin. He should certainly not have stood there with his thumb up his butt chatting with a 911 dispatcher for more than a minute after losing sight of Martin. And that's why I feel the claim of self-defense was not justified.
Addendum: I read an interesting blog article
that referred to an interesting martial artist making a remarkably stupid decision to walk through Central Park late at night, because he had a black belt in karate. The parallel to the Zimmerman thing is interesting - he was confronted before he even got 100 yards in and was injected with a hypo by one of his three attackers. But he made up for it by retreating, as fast as possible, and got out essentially unharmed. I can't help but wonder what would have happened if Zimmerman had been sensible enough to run once Martin confronted him. As it stands, he was rather foolish, apparently thinking that his status as the head of the neighborhood watch and the fact that he was armed would be enough.
If Martin had been seriously trying to kill him, he would have been dead, gun or no gun.