I believe that the most important aspect of human morality is honestly.  So I've always been... reluctant... about telling my kids Santa is coming.  
On the one hand I want them to enjoy the anticipation I did as a kid; on the other, I feel very strongly about not lying to or otherwise misleading my children.

Unfortunately (or maybe fortunately) I've gone along with the cultural norm of telling my kids that "Santa is coming" when Christmas time rolls around.  Though usually when the oldest asks or talks about details like coming down the chimney, or living at the North Pole, or who were Santa's parents, I just stay quiet and let some one else do the answering, which is admittedly a moral cop-out.

Given that 

- My kids already believe in Santa, 
- I want them to enjoy the anticipation and mystery of Christmas, 
- I am not open to lying to them 

How should I handle myself when my kid asks questions about Santa, makes comments about Santa, or someone else tells them something false with relation to Santa?