There's a world of difference between true and honest, and between false and deceptive. Something can be true without being honest, and false without being deceptive -- and vice versa in both cases.
Most people think lying is wrong, and what we generally object to isn't saying things that are false but saying things that are deceptive. There are many circumstances in which making false statements is acceptable, or even laudable. For instance, fictional writing is all, by definition, false, but unless the author tries to disguise his fiction as fact no one is bothered. There are even many games that require players to make false statements and to be deceptive within the context of the game -- no one has moral objections to bluffing in poker because that's how the game is played; bluffing is expected in general, even if people are deceived in a particular instance. At an even more serious level, criminal defense lawyers are expected to be deceptive (within the bounds of the law (the "game")) and to put on the most vigorous defense possible for their client. This moral allowance -- if you want to call it that -- helps ensure that the innocent aren't wrongly convicted.
On the flip side, everyone knows it's possible to speak the absolute truth and still be deceptive. For example, imagine the following exchange:
A: I think everyone should give $100 to the soup kitchen!
B: That's a good idea, are you going to?
A: Everyone should!
A hasn't actually answered B's question, but it would be easy for B to infer from A's response that A is going to give $100. The question of deception hinges on whether or not A actually implied that, given the context. "Everyone should" doesn't mean "I will", and it only directly implies "I should". Most people in B's place wouldn't be entirely decieved and would recognize that A left himself some wiggle room, but people would also be restrained by common courtesy from asking the follow-up: "I agree they should, but will you?"
Attempts to clarify and eliminate wiggle room are generally seen as rude, partly because they imply that the questioner doesn't trust the original speaker. More than that though, I think it's generally recognized that if someone doesn't want to give a straight answer, that itself is an answer.
As for the ethical question, false statements are clearly not prima facie immoral. Deceptive statements may be immoral, given the larger context and the rules governing the game being played. Spies and secret agents must lie to protect their identities, but they're expected to do so and only condemned if they deceive the wrong people and betray the group they should be loyal to. Hence, an American spying on America for Russia is immoral, but an American spying on Russia for America is acceptable.
(The context is even larger than that, however: an American may think a Russian spying on Russia for America is moral because America's goals are more noble. Ends must almost always be used to justify the means when moral questions revolve around international affairs and sovereign nations. If you don't agree, consider people who deceived the Nazis in order to shelter Jews during the Holocaust.)
Finally, let me compare deception with killing. We have a special term for immoral killing: murder. We need a similar word for immoral deception. Not all killing is immoral, and neither is all lying or all deception. The Bible puts it pretty clearly:
Exodus 20:16 You shall not give false testimony against your neighbor.
Not all lying or deception is condemned here, but only false statements of truth in a formal setting against someone you should be loyal to. That's not a very high standard when you really consider it, and I think the requirements for moral behavior are often greater, depending on context. This command gives us a good foundation to build upon, though.
The issue of honesty is actually quite mysterious, and there is no existing theory to explain why humans are so honest. In any given instance it can be incredibly advantageous to be deceptive, and humans don't generally give long-term consequences much weight. It has been proposed that enforcement of long-term consequences for deception can explain honesty, but simulations have shown that the costs of enforcement are higher than the cost of deception over time. I have a thought on how to solve this shortcoming, but I'll write more about it later.
Did you cover the category that is sometimes referred to as "what he/she doesn't know won't hurt him/her." Not lying or deceiving, but not saying anything.
Example: man's cheating on his wife. He never "lies" to her about it since she never asks the question: "Are you cheating on me". He just never mentions it. Assuming he doesn't lie or deceive about what he is doing in his spare time (I'm going golfing, I'm working late), it is conceivable (if improbable) for him to have an affair without ever *verbally* lying or deceiving his wife.
Notice that it's OK by the Ten Commandments to bear false witness FOR your neighbor, just not against him. IIRC, however, the teachings of Christ prohibit lying more generally. Am I wrong?
DM: I think silence can be deceptive too, if there's a presumption of fidelity.
X: Literally I see your point, but the phrase "against your neighbor" generally implies "against anyone" (and, further, almost certainly does so under NT teachings). Testimony for someone is generally against someone else (even in criminal proceedings). The correct interpretation appears to be that you shouldn't tell falsehoods in formal/serious settings to help yourself or hurt someone else.
As you (IMHO) correctly point out, Jesus' teachings raise the bar quite a bit with regards to many OT laws: from divorce to lying to spiritual purity.
I'd like to see some discussion on the vast volume of deception that seems to go on around the blogosphere (and elsewhere, I'm sure). Take an issue, any issue. There will generally be one set of people that take one side, and another set of people that take the other side. Each set of people will either do their own research and reading to find arguments that support their side. The trouble is, eventually both sides get to the point of factual overload that although the "facts" start to become contradictory, neither side recognizes it of themselves. And the cycle continues, with one side or the other recycling as truth, what have now become self-evident as lies.
Confusing, I know. Case in point: WMD in Iraq. The wide left continues to push the argument that the Bush administration was adamant that Iraq was an "imminent" threat. Numerous instances where it was proven that nobody in the Bush Admin ever said the threat was "imminent" - meaning right now - are ignored, and the meme continues. Most people pushing the case are adamant in their own side's infallibilty and ignore the facts on the other side, considering them "nitpicky" or "suspect".
In these cases I don't think most people are lying to intentionally deceive - I think people can become so caught up in the cause they serve that they mold the natural laws of debate to fit their world view.
And it truly happens both ways - for the other direction (just to give equal time), the reality/fantasy of "Global Warming" is another controversy where in this case the wide right pish-tosh's environmentalist claims of Global Warming, in spite of scientific claims to the contrary. They create their own reality in which they're not actually lying - they're deciding to exist in a parallel universe where they're actually correct.
Discussion? Bricks? Tomatoes?