Judith Miller has worked off and on reporting on Saddam, terrorism, and WMD in the Muslim World. I believe at some point well prior to this Niger business that she learned Valerie Plame was CIA and the two were acquaintances. When Kristof produced his story it would be only natural that he would discuss it with Miller. Second, and I think the most important point missed by analysts was the February 2002 Defense Intelligence Agency report titled "Niamey signed an agreement to sell 500 tons of uranium a year to Baghdad." It was this report that spurred so much interest in Niger in the first place.
But how was it revealed that Judith Miller knew about Plame if she never published the fact? (I'm not defending her here, I'm just trying to understand where she fits in with regard to Kristof).