On November 28, 2004, the Washington Post reported:

Fitzgerald wants Miller to testify about her talks with Libby, even though she never wrote a story about Plame. Libby has signed a letter saying he waives his anonymity, but Miller still refuses to talk. Her position is that no such waiver under pressure from a prosecutor can ever be voluntary.

As I pointed out here, Fitzgerald knows who her source is. Look to the subpoena to Judith Miller (from this DC Court of Appeals decision (pdf) upholding Cooper and Miller's contempt orders:

In the meantime, on August 12 and August 14, grand jury subpoenas were issued to Judith Miller, seeking documents and testimony related to conversations between her and a specified government official “occurring from on or about July 6, 2003, to on or about July 13, 2003, . . . concerning Valerie Plame Wilson (whether referred to by name or by description as the wife of Ambassador Wilson) or concerning Iraqi efforts to obtain uranium.”

Note that it requests conversations between her and "a specified government official" - that's Libby. And that the time period is the week before Novak's article was published. And, it refers covers conversations pertaining to Valerie Plame Wilson by either name or as the wife of Joseph Wilson.

The question is what did Libby tell Miller that she is so intent on concealing that she's willing to go to jail over it...and does it implicate Dick Cheney, other members of his staff and/or Karl Rove?

Another piece, the Chicago Tribune, July 7, 2005:

Fitzgerald, the U.S. attorney in Chicago, has said for months that his investigation has concluded except for the testimony of Miller and Cooper. On Wednesday, he told the court that with Miller's refusal to testify, "we are having this whole thing derailed by one person."

Judith Miller told Aaron Brown on Newsnight (CNN Aaron Brown Newsnight, CNN October 25, 2004 Monday (Lexis.com):

I felt that I didn't want to start to go down the road of testifying about someone who may or may not be a source, because, at this point, the focus of Mr. Fitzgerald's inquiry has been on one person. But, as we've seen from Matt Cooper, if you make a deal to discuss that one person who may or may not have given a voluntary waiver, what about what happens when Mr. Fitzgerald's target of interest or person of interest shifts?

And then there's another person and another person who comes under suspicion. And, eventually, somebody might actually get to one of your sources, if they haven't already. I just decided that the position has got to be, if I promised someone confidentiality, whether or not he was a source on a particular story, I'm not going to go in and testify about what that person told me. Otherwise, I can't do my job.

I think the issue is, did Libby tell Miller about a meeting he was at with Dick Cheney - or other members of the White House Iraq Group - at which a plan to discredit Joseph Wilson using his wife's CIA status was hatched? It's seeming more and more likely.

And even if Libby didn't tell her about a meeting, he had to have told her something that puts someone else in jeopardy, or her statement to Brown doesn't make sense.

Billmon, on the other hand, thinks Judith Miller might be the leaker.

The issue isn't whether Miller is covering up for a source, the issue is whether she was a source for Operation Slime Wilson.

Balloon Juice thinks Judith Miller should be forced to tell. One of his commenters responds:

Suppose she is in jail six months from now. How will you "force" her to talk, then? Play loud music in her cell 24 hours a day, perhaps?

Maybe they'll play Glenn Frey's Smuggler's Blues:

From the Office of the President
Right Down to Me and You

Update: I just saw this from today's Newsday over at Digby's - Miller was in Washington on July 8 and met with a Government official:

The disclosures also highlight the special prosecutor's interest in jailed New York Times reporter Judith Miller and her meeting in Washington with an unnamed government official on July 8, 2003, the same day, according to the disclosures, that Rove spoke with Novak.