Lying by Omission is not...
You don't have to share every detail of your life with everyone, especially when they are not asking for it. For example, if you are afraid to tell your parents about your religious beliefs, you don't have to (you also shouldn't be afraid). If you feel compelled to tell a girl walking by that you want to have sex with her, you don't have to (= shouldn't). If you are gay and not ready to come out of the closet, you don't have to (unless you have a girlfriend or boyfriend). If somebody expresses an opinion, and you don't want to reply with your own, you don't have to. However, in a perfect world, we would all be able to express ourselves freely to anyone. You also don't have to tell people what you think of them, just because they are in front of you. That's just being mean.
What about so called 'little white lies,' telling your wife she looks good when she looks bad? I don't believe it. You married your wife. She's always beautiful. End of story. If her new haircut sucks, tell her the truth.
A Simple Test
Lying is intentionally giving someone wrong information, while lying by omission is intentionally leaving out information that someone deserves. Lying by omission, I think, is wrong when you are asked a question and don't fully answer it. You leave out facts that are relevant, details that the person asking would probably want to know. However, a person doesn't have to ask the question. Some people simply have a right to information, whether they ask or not. When? A good test is to put yourself in the other person's shoes. Would you want to know? Then yes, pray tell sir. The golden rule shows up again. The fact that your information will hurt someone is not the relevant consideration, although it helps to think about. Do they deserve the truth? - that's more relevant.
"Nazis. I hate these guys." -Indiana Jones
Immanuel Kant would say the same thing, considering this ridiculous example that breaks down the rule that lying is always wrong. Nazis show up at your door. "Are you hiding Jews?" they say. "No," you say. "I'm not hiding any Jews." But, secretly, in your head, you're thinking: "I'm hiding people, but not Jews; I don't consider them 'Jews," as you do. Well, that's very crafty wordplay, but clearly that's lying by omission. If you put yourself in their shoes (scary), they clearly want to know that information. And of course, looked at in hindsight with all the information about what the Nazi's were up to, it's the right call. You should not let Nazis kill people. When faced with radical evil, the rules bend I suppose. Statistically, a case like this will not happen to you. It's just a philosophical thought experiment which helps to prove the rule it was intended to break: that lying is wrong (except when Nazis are at your door and you know the future).