The Washington Post has a useful backgrounder on the law in question, the Intelligence Identities Protection Act, enacted in 1982 after former CIA agent Philip Agee (who now lives in communist Cuba) published a book and various articles revealing the names of undercover CIA agents in an effort to sabotage U.S. intelligence activities. Even if Plame does turn out to be a covert agent, revealing her identity wouldn't automatically be illegal:The law enacted to stop Agee and others imposes maximum penalties of 10 years in prison and $50,000 in fines for the unauthorized disclosure of covert agents' identities by government employees who have access to classified information.
The statute includes three other elements necessary to obtain a conviction: that the disclosure was intentional, the accused knew the person being identified was a covert agent and the accused also knew that "the United States is taking affirmative measures to conceal such covert agent's intelligence relationship to the United States."
Taranto concludes: "In order to violate the law, in other words, one must disclose a genuine secret. Was Plame's association with the CIA a secret? As we said yesterday, the CIA's blasé attitude toward Novak's inquiries suggests not."