Sam Stein at HuffPo lays out the legal issues I referenced earlier. The big question is whether or not Sherrod is considered a public official. If she is the 1st Amendment offers broader protections and she'll have to prove that Breitbart acted with actual malice. If she's a private person she just has to demonstrate that Breitbart acted negligently.
I think if you hold a director level position at the U.S. Department of Agriculture and are prominent enough to be asked to speak at an NAACP banquet then you are probably a public official for purposes of the 1st Amendment. But even then I don't think proving that Breitbart acted with actual malice (knowledge that what he was publishing was false, or that he acted with reckless disregard for the truth) is out of the question. Breitbart's behavior throughout this episode has been nothing short of appalling.