My rule of thumb is something like: if I have to worry about it being uncovered, it's a (morally wrong) lie. If I don't care if it's uncovered, it doesn't really count as a lie. If I'm sick and frustrated and angry and tired and someone asks me how I am and I answer "I'm OK", it may be an incorrect representation of reality, but not one that I will try to protect from further investigation.

That conveniently extends to Santa Claus - if my son investigates the Santa Claus story and says it's false, I won't come up with more details to cover it up (unless I can come up with something sufficiently ludicrous that I can say with a straight face).