From the tone and tenor of the letter, it sounds to me that Fitz is not interested in Libby, but Miller.
Check out the NY Sun article the Fitz letter came from. It's titled "Prosecutor Thought Libby Deliberately Failed To Intervene On Reporter's Behalf"
http://www.nysun.com/article/20936?access=785492
I don't think the target of an investigation (you proposed Miller) is required to testify before a Grand Jury. Is that correct?