The creativity and imagination of a child is thrilling to witness. It is great to see how the mind works when unfettered by social convention, something only the most creative among us seem able to carry into adulthood. Nobody has to tell a child to "think outside the box;" that is the only way they know how to think.
And yet, it is difficult to imagine that lying to our children simply because we find it entertaining is ethically sound. And yes, this includes the socially acceptable Santa Claus lies. A child is supposed to be able to trust his or her parent, and we are right to object when a parent violates that trust. I suppose an argument can be made that it may be beneficial to lie in situations where doing so helps to protect one's feelings (i.e., "white lies"), but that is not what we're talking about here. Adopting the Santa charade is not about protecting the child's feelings; parents who do this usually do it because they enjoyed it as children and now enjoy providing their children with a similar experience.