Simply not credible. No one would sit in jail, even in Alexandria, for three months simply because she didn't feel like picking up the phone and calling Libby to hear him say the waiver directly.
She has decided for reasons of her own that right now is the best time for her to come out and face Fitzgerald's questions, and is just using this story about the waiver to fool people into thinking otherwise.
She may have been pursuing other tacks during this time, for example seeing if powerful republican friends could somehow help her out, maybe public outrage would somehow come to her rescue, etc., and has now finally concluded that none these other possibilities will pan out.
Or perhaps she has finally managed to figure out through sources just what Fitzgerald knows, and armed with that knowledge will take her chances with the grand jury hoping to employ some deft Clintonisms while continuing to protect whatever it is she doesn't want to reveal.
Good possibilities.... OR... see my #51 above for my current candidate.
Fitzgerald was going to file a criminal contempt charge against Miller when the grand jury term expired, so Miller and the Times decided to testify. They are trying to spin and obfuscate the details of the release, but Miller was going to be charged with contempt on Monday if she didn't testify tomorrow.