I've argued that typically the person who is hiding Jews from Nazis and is asked by a Nazi if there are Jews in her house tells the truth, and does not lie, if she says: "No." That argument might be wrong, and even if it's right, it probably doesn't apply to all cases.
So, let's think about the case of the Nazi asking Helga if she is hiding Jews, when she is in fact hiding Jews, and when it would be a lie for her to say "No" (i.e., when there isn't the sort of disparity of languages that I argued is normally present). The Christian tradition has typically held lying to be always wrong, including thus in cases like this. I want to say some things to make it a bit more palatable that Helga does the right thing by refusing to lie.
The Nazi is a fellow human being. Language, and the trust that underwrites it (I was reading this morning that one of the most difficult questions in the origins of language is about the origination of the trust essential to language's usefulness), is central to our humanity. By refusing to betray the Nazi's trust in her through lying, Helga is affirming the dignity of all humans in the particular case of someone who needs it greatly--a human being who has been dehumanized by his own choices and the influence of an inhuman ideology. By attempting to dehumanize Jews, the Nazi dehumanized himself to a much greater extent. Refusing to lie, Helga gives her witness to a tattered child of God, a being created to know and live by the truth in a community of trust, and she he gives him a choice whether to embrace that community of trust or persevere on the road of self-destruction through alienation from what is centrally human. She does this by treating him as a trusting human rather than a machine to be manipulated. She does this in sadness, knowing that it is very likely that her gift of community will be refused, and will result in her own death and the deaths of those she is protecting. In so doing she upholds the dignity of everyone.
When I think about this in this way, I think of the sorts of things Christian pacifists say about their eschatological witness. But while I do embrace the idea that we should never lie, I do not embrace the pacifist rejection of violence. For I think that just violence can uphold the dignity of those we do violence to, in a way in which lying cannot. Just violence--even of an intentionally lethal sort--can accept the dignity of an evildoer as someone who has chosen a path that is wrong. We have failed to sway him by persuasion, but we treat him as a fellow member of the human community by violently preventing him from destroying the community that his own wellbeing is tied to, rather than by betraying with a lie the shattered remains of the trustful connection he has to that community.
I don't think the above is sufficient as an argument that lying is always wrong. But I think it gives some plausibility to that claim.