The NYTimes has an interesting graphic on the subject: "Who Spoke to Woodward"? Don't know if the link will work without a subscription. While it's a good guessing game, it also shows the pervasive bias of DC, and perhaps the emotional reality of working in a bureaucracy.
Take a look at the graphic: it lists only "principals", no personal secretaries as they used to be called (remember Rose Mary Woods, Nixon's secretary?) or personal assistants/executive assistants as seems to be the modern nomenclature. I'd wager that the majority of these people have one or more people who knew about Ms Plame. But people never see these people when they discuss bureaucracy. I'd love to know if Woodward shares this vision problem. When he was hanging around the White House and ran into Mark Felt, was he really chatting up the secretary?
It's possible the answer is "no". The emotional reality seems to be that most PA's are so loyal to their boss (think Woods and Fawn Hall, Ollie North's secretary) that they're more discreet than their boss. (And sometimes more capable, though that's a topic for another day.)