Lying to your kids: Necessary evil or just plain evil?

So I was reading this thing on TODAY Moms that was talking about moms lying to their kids--about a variety of things, big and small. A lot of the moms commenting on the article were saying that they would NEVER lie to their kids, that it was wrong, etc. etc. It got me to thinking about lying to your children. Parents spend a bulk of their children's childhoods telling them "DO NOT TELL LIES!", but then they lie to their kids about any number of things: from big things like Santa, Tooth fairy, Easter bunny, the death of pets, etc. to little things like whether you're REALLY out of ice cream or if the ice cream truck is just a "music truck". It got me to wonder: When it is okay to lie to your kids? I asked John if he thought it was okay to lie to kids and he said "Sometimes". No help there. I would ask my parents, but I know what they would say. My mom would say "No, it's NEVER okay to lie to your children!", but she was the main proponent of Santa Claus, even going so far as to perpetuate belief in Santa Claus till I was in high school! (In fact, she STILL acts like she completely believes in the jolly old man) My dad would say "It's ABSOLUTELY okay to lie to your kids!" He lied to me fantastically and often when I was a child. My favorite was when I was 3 or 4 and I asked him why the moon changed shape. He told it was God winking at me. Now according to the parents that commented on TODAY Moms, my dad should have taken the time to explain to me (a 3 or 4 year old) the phases of the moon, etc. I would have known about it early (actually prolly not, since I prolly wouldn't have understood what the heck he was talking about), but it would have made my childhood a little less fun. Is it okay to lie about some things in order to make sure your child stays innocent in their childhood as long as you can? Or should you "practice what you preach" and NEVER lie to your children, even if it causes them to grow up too fast? I know there has to be a fine line between the two--but where should you draw it? And how do you KNOW where you should draw that line?