"The law protects the identity of previously covert operatives, as well"
I think there's a five year limit on that, and WIlson/Plame had passed that 'statute of limitations.'
There is a time limit, but I am not sure you are correct that it had passed, and I doubt that we will ever get the details on that because they aren't going to discuss the details of her covert service.
One thing about all this is that both the media and the prosecutor are dancing around the question of whether she was a covert agent at all because they know it's illegal to disclose the identity of a covert agent. Nevermind that the law does not apply to the media and the fact that her cover is already blown. The media will not discuss the question of whether she is covert because of the law against disclosure, and also becuase they don't have the details since the government won't talk about it.
And that, by the way, is one of the reasons why this whole sordid episode happened to begin with. Someone from the CIA could easily have told Novak, "Hey you idiot, don't publish this because she's a covert agent."
But if they'd done so, they would have been violating the law, so they just settled for begging him not to print the story, without explaining why. He obviously was not smart enough to read between the lines.
My suspicion is that the time limit had not passed, though, because if it had, there would have been no grand jury.