In practice, most journalists refuse to identify their sources under any circumstances at all, even when it's clear that those sources deliberately lied to them. But should that be the standard? Or is the profession — and the rest of us — better off if sources know that they run the risk of being unmasked if their mendacity is egregious enough to become newsworthy in its own right? I'd say the latter.
Why reporters go to great lengths to conceal their sources is obvious; without such protection, many whistleblowers and the like will never come forth to reveal government wrongdoing if they cannot be protected from retaliation. However, the period leading up to the war in Iraq was marked by numerous false claims being "leaked" to sources in the news, claims that could not be challenged because their origin was uncertain and the sources risked no shame, humiliation, or criminal punishment for telling lies to the public trough the media. Clearly, the defensibility of a source's privacy can and should rest upon the veracity of the claim they make. Media sources should know that if their claim is revealed to be an obvious and malignant lie, they will not be protected from the harsh light of public opinion or the possible sanction of criminal punishment. This is so completely obvious a conclusion to arrive at from the lessons of 9/11 and the Iraq war that it's impossible to imagine members of the media agreeing, but thus far, they do not, and ABC News continues to refuse to out their sources.