"it's not for any journalist to decide when it becomes okay to expose a covert agent."
Maybe you should read the law and classification.
Plane did not meet the qualifications as a "covert agent."
She was working at Langley which last time I checked is not "foreign" soil. LOL
Taking little snippets from the great body of statutory and common law, and then claiming that anything that doesn't violate the particular snippet you cited is legal, is a silly exercise. Trust me, if you happen to know the identity of a former covert CIA agent whose role has not been publicly disclosed by the CIA, you'd do well to keep it to yourself.
In the Plame case, the journalists in question may be able to make a plausible argument that they didn't realize the agent's identity was still secret, since they were given the info about her former role by a high-ranking government official. The high ranking government official, however, is likely to be in deep doo-doo -- though from a practical standpoint, his prominent role in the Bush Adminstration will mean he probably won't be prosecuted.