Mark Shea dives in again to the question as to whether the Live Action operatives who conduct sting operations against Planned Parenthood are exempt from the Catechism's prohibition against lying. Short answer: no.
It seems pretty clear: we are not permitted to tell deliberate lies, not even when we think we have good motives. Not every act of deception is a lie, of course, and thus not every act of deception is a sin--but when we tell someone, on purpose, something which is not true it's pretty hard to cover that under some sort of an exemption.
In the discussions about this matter, people have brought up undercover journalism and undercover police work--and even the "mystery shopper" whose purpose is to check up on customer service in a way that seemingly tricks employees. While some of these things don't necessarily involve outright lying, to the extent that any of them do they are not any more morally permissible than any other sort of lying. However, it would be necessary to scrutinize such situations rather deeply to determine whether such pretenses actually rise to the level of lies, and in fact, at least as far as police work goes, I believe that police officers must be rather careful to avoid telling the kind of outright lies that will make prosecution of the alleged criminal difficult or impossible from a legal perspective.
Most situations involving pretense also involve people who are aware that a pretense is going on. Actors, writers, and others who craft or perform fiction don't have to worry that they are lying to audiences when they pretend to be "Mr. John Smith," or when they write that Alice screamed as she entered the living room and discovered the body; viewers and readers know that the actor pretending to be Mr. Smith is not any such person, and that the account of Alice's subsequent adventures as the person unjustly suspected of murdering her Great-Aunt Hildegarde for the sake of her inheritance is not meant to be a history, but merely a pleasurable escape from reality. Even in the hypothetical "mystery shopper" case, it is quite possible that employees of the company have been told in no uncertain terms that the store employs such people, and that for their own sake they should act as though every customer is evaluating the store's customer service--as, indeed, every customer may be, though only a handful will be paid to do so.
But are Live Action's video tactics covered under these sorts of provisions? I hardly think so. The operatives who enter the abortion clinics are pretending to be a pimp and a prostitute, engaged in prostituting underage girls who will then need the clinic's services should they become pregnant. They can't enter into this deception without lying (though it is interesting to speculate as to whether, if they could do so, they would then be able to conduct their "sting" without any moral issues). They are not merely withholding the truth from those who don't deserve to have it. They are not practicing reticence, nor engaging in theatrical performance. This is not some harmless prank of the "Candid Camera" variety--and the fact that the harm meant is directed towards a palpably evil organization does not change the morality of the act of lying.
To say this is not to doubt the courage displayed by these people, nor the fact that their motive is to put a dent in the lucrative and evil abortion industry--but evil means don't become good because the people employing them are courageous or because the people they are directed against are engaging in evil; we've been over all of that in the torture debate, haven't we?
The truth is that no matter how much we might wish to believe that Live Action's tactics are morally sound, we ought to know better than that. Truth be told, abortion will only end when it is fought against with the truth--that human beings are, from the moment of conception, intrinsically worthy children of God, made in His image, and worthy of love and protection. But we can't illustrate that truth very well if we treat some of God's children as objects whose only value lies in their being tricked into revealing Planned Parenthood's evil secrets.