Update: July 5
The lack of Zimmerman's DNA on some of Trayvon's effects is seen as helpful to the prosecution.
But a medical examiner brought notes into the courtroom today. The notebook must be made available to everyone (the defense), and this behavior by a witness may hurt the prosecution.
If Zimmeran is convicted "only" of voluntary manslaughter, under Florida law he still could get 15-20 years since Trayvon was under 18.
Jonathan Capehart has an important column Sunday in the Washington Post, "5 Myths about the Killing of Trayvon Martin", link. The column may give the defense a little more ammunition, even for acquittal. The 9-11 emergency operators cannot give Zimmerman direct orders without liability; hence they worded things indirectly, "We do not need you to do that."