This per Newsweek:
Oct. 10, 2005 issue - New York Times reporter Judy Miller broke her silence and agreed to testify before a federal grand jury last week. This followed tense, often acrimonious negotiations that began after special Justice Department prosecutor Patrick Fitzgerald signaled he intended to reimpanel a new grand jury—a move that could have kept Miller in jail for another year and a half, say two lawyers close to the case who asked not to be identified because of the sensitivity of the talks...Fitzgerald indicated he would not let the matter drop when the grand jury, investigating the leak of covert CIA agent Valerie Plame's identity, expires in late October. Instead, he would keep his long-running probe open with a new grand jury. The sobering prospect spurred fevered negotiations among lawyers to find conditions that would satisfy both Miller and Fitzgerald.
http://www.msnbc.msn.com/id/9558011/site/newsweek/My recollection is that Cooper and his team also were concerned about approaching Rove to discuss super duper special waiver of confidentiality also because that might leave Cooper open to obstruction of justice charges. They also negotiated with Fitz on this if recollection serves. So I don't think there's anything particularly significant in Miller avoiding that charge if she had interaction with Libby to discuss a specific waiver. (Although I still think the waiver issue was bogus. The element of coercion is there once Libby was directed to sign the blanket waiver, reiterating the waiver in specific regard to Judy doesn't change that really. Especially if, as Libby's lawyer claims, they addressed that specific issue a year ago.)
What is more significant is that the basis of her testimony was limited (as with other reporters, but Judy is unlike other reporters in her interaction with and promotion of the administration's specific WMD claims).
But the compelling reason for talking is that Judy reportedly was threatened with the prospect of doing considerably more time in jail, since Fitz played hardball with her. A few months in jail is one thing. Adds a bit of sheen to her new image as a first amendment "martyr." A year or more in jail is another natter. Fitz wanted her to talk, not just keep her in prison. Otherwise the threat would have been criminal contempt charges.