Cacciaguida has explained his take (which is congruent with mine) on Santa. I have long since decided I will NOT lie to my kids about the existence of Santa, because I was really hurt when I found out that I had been lied to about Santa. I couldn't understand why my family had collaborated to make a fool of me. I had asked my mother directly if there was a Santa, and when she told me yes, I had valiantly defended Santa to all my friends who had figured out the truth, because I trusted my mother's word. The only thing I took away from the whole business is that it was somehow funny for the family to lie to the little child because they knew she would believe what they said.
No Santa for my kids, that's for sure. But just wait until my in-laws find out - I dread this conversation. O.O.'s parents are unable to distinguish causation from correlation, and his aunt raised her now wacked-out kids in a new-agey fashion which included no Santa, calling parents "Joan" and "Ted," etc. So in their minds failing to tell kids about Santa is tied up with an irresponsible level of "letting the kid decide what to believe." Argh.
So maybe if you've lied to your kids this year, you can take them aside after Christmas and explain that while Santa isn't a person living today, he is the remembrance of St. Nicholas, who was so kind to all the children when he lived on earth that parents still tell their children about him and give gifts as he would have today. Tell them that when they were younger, it was hard to explain that St. Nick had lived a long time ago but was still bringing them presents, so you told them he was a man who lives today and flies around the world. Now that they're older, they can understand the more nuanced situation.
Sure, it's presumptous of me to recommend to others how to raise their kids. But I think if you have told your kids their is a Santa, you should at least tell them WHY and relate it back to a larger point about Christmas, rather than just letting them figure out that they've been duped.