I B,
I'm not a lawyer but I believe the state will be allowed to introduce other versions of the story into the trial - a la Judge Ito.
Remember, Zimmerman waived his right to counsel and fully cooperated with the police that night and the next day. That is a long time to go without substantial sleep after a trauma and there is just going to be some inconsistencies in what he said. Likely nothing big, but I think the prosecution is going to be allowed to hammer away at a thousand insignificant nails.
As long a Zimmerman didn't show his gun until after he was knocked down then its going to be hard to get a conviction.
The other wildcard is Trayvon's "girlfriend". Strangely I'm not seeing her testimony although it was all over the news before he was arrested. From what I remember she wasn't even contacted until 2 weeks after Martin's death. She's going to be treated as a saint although what she remembers is highly suspect and likely influenced.
Just saw where Trayvon's Mother is being given 8 months leave of absence from her job. Gawd I hope the trial doesn't drag out that long.