The New York Times, by way of The Orlando Sentinel, has a few more details about George Zimmerman's account of his fight with Trayvon Martin:
Zimmerman said that Trayvon had punched him and then repeatedly slammed his head into the sidewalk in the moments leading up to the shooting....
In the 911 calls that have been released, Mr. Zimmerman is heard deciding, against a dispatcher’s advice, to follow Trayvon, whom he deemed "up to no good."
In Mr. Zimmerman’s account to the police, he returned to his S.U.V. after he was unable to find him. Trayvon then approached Mr. Zimmerman from behind and they exchanged words. Then, Mr. Zimmerman said, Trayvon hit him hard enough that he fell to the ground — which would explain what Mr. Zimmerman’s lawyer, Craig Sonner, has said was a broken nose — and began slamming his head into the sidewalk.
This account is consistent with Zimmerman's injuries and the report of an eyewitness who called 911, although that man (identified in the papers only as John) did not see how the fight started. It is also consistent with what Martin's girlfriend, who was talking to him on his cellphone right before Zimmerman shot Martin, said she heard. Judging from that conversation, Martin was understandably worried about the guy in the SUV who was tailing him and decided to confront him.
While it's clear that Zimmerman created the circumstances that led to the fight, it's not clear who made the first aggressive move after the two of them "exchanged words." By Zimmerman's account, Martin threw the first punch, but for all we know Martin was responding to what he perceived as a potentially deadly threat—a plausible reaction if we assume that Zimmerman displayed his gun or Martin caught a glimpse of it. In other words, something similar to the scenario outlined by Julian Sanchez, in which both Martin and Zimmerman reasonably feared for their lives, may actually have happened. If so, Zimmerman would still be responsible for needlessly setting these events into motion. But if his account of how the fight unfolded is true, his use of force could be justified under Florida law—not because of the right to "stand your ground" established in 2005 but because of the right to use deadly force when you reasonably believe you are "in imminent danger of death or great bodily harm" and have "exhausted every reasonable means to escape."
Addendum: Joe M notes in the comments that Martin's girlfriend (as quoted by The Guardian) "thinks she heard Zimmerman push Martin 'because his voice changes, like something interrupted his speech.'" If so, it may have been Zimmerman who started the fight. When I said Zimmerman's account was consistent with hers, I had in mind the brief exchange between the two men that preceded the fight.