More important is the statement of a witness to the fight. Also, there are some unflattering statements of two acquaintances of Zimmerman.
Update: This post originally linked to an AP story on the WaPo site, but now the link goes to the WaPo's own story by Sari Horwitz and Stephanie McCrummen. On page 2 is the disappointing answer to the question I've been asking:
A police report also concluded that the voice screaming in the background of a recorded 911 call placed by a resident was Zimmerman, "who was apparently yelling for help as he was being battered by Trayvon Martin."
But an FBI audio analysis of that crucial call could not determine whether it was Martin or Zimmerman who was screaming, because of the poor quality of the recording and the "extreme emotional state" of the person screaming, an FBI report said.
A new AP story by Kyle Hightower and Mike Schneider is here, emphasizing that the evidence is "a mixed bag."