Well, I think that lies are separated into constructive and destructive lies.
For instance, someone bumps into you and asks you if you're a virgin. You have nothing to gain from telling that person, but they coerced you out of information, or misinformation. You can either answer, or not. If you don't answer, it's basically like saying you're a virgin, in most cases anyway.
So why should junk information that does no harm be considered immoral? There's a lot of things that can be done with the most trivial of information, so you would be wise to stay on the safe side and not allow anyone into your personal matters that you wouldn't entrust them to.
I can go ahead and say this: As a species that relies heavily on information, reliable information is imperative to our survival. It's something along the lines of biological leash: Should a species spread generally more misinformation than genuine information, it will not thrive very well. The fact that we have language in itself means that humans are generally trustworthy. The fact that misinformation propagates about as well as genuine information doesn't make this statement untrue. It's because humans are dumb. Honestly so, but dumb.
Anyway, I digress: So what about constructive and destructive lies? A destructive lie would be to give misdirections. A destructive lie would be to tell your uncle that no, there isn't a hive of bloodthirsty wasps in that motorboat's powerhead.
Now your child's goldfish dies. I'll leave it to you: You can either tell it the truth, or you can tell it you're gonna flush it back in the ocean so it can wake up from hibernation. Whether or not this is constructive to the growth of a child, I'll also leave to you.
A better example is telling your wife she doesn't look like sausage casing in that dress. Putting your values in a hierarchy, then having her shut up and be jolly for one evening is better than ruining the day, coming too late or create a potential menace. The lie is constructive. We will continue to hate each other anyway and men don't really care about other men's wives.
Anyway, bottom line, I think honesty is really, very important. I will not attempt to philosophically prove that lies are immoral. I just believe it's not the province of law, which is the important thing to note - but the province of morals, and the morals of lying are strictly contingent upon the lie in question.