Newsweek quotes a CIA source saying it was the threat of further jail time that made Judith Miller start talking:

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. What Miller wanted was a direct, personal assurance from Libby that he had no problem with her testifying about two conversations they had in July 2003. That finally came on Sept. 19, in what participants described as an "awkward" four-way conference call that included Libby, Miller (patched in on a jailhouse phone) and their lawyers....