I didn't want to do the whole Santa thing, but my father was really into it, and I didn't want to ruin it for him (he would have been pretty disappointed). So I never bring up Santa, if my son does, we will talk about him a little, but I never go out of my way. I haven't said if he is real or not (he hasn't asked, he's only 4 and just believes). With presents, he always talks about Santa bringing presents, but we don't mark presents as who they are from, we just wrap them in the same paper for each person (it is just the three of us). I do tell him if they are from different people, but I guess he just thinks what we buy (he only gets maybe three things from us for Christmas each year), is from Santa. I don't care, we don't keep it a secret, and will say things that make it sound like it is from us. Anyway, my point is, you don't have to say anything. If he asks, then you can answer that question with a simple answer (If you believe he is, then he is, is a good enough answer), but it probably won't even come up. Also, we went and bought gifts for Toys for Tots, and I specifically said we were getting gifts for kids that weren't getting presents, and he didn't put that together with Santa bringing gifts for all kids. If he does next year, I will explain the spirit of Christmas, and how we are passing that on, especially by giving those gifts.
Sorry, one other thing, I was just reading some of the other comments, and I totally tell my son that movies and cartoons aren't real. Not every time he watches something, but it does come up a bit. My son needs to understand the difference because sometimes things he sees upset him, and him knowing that they aren't real helps him. He also understands enough of the world to ask a lot of questions about what he sees on tv and in real life, and he has brought it up the realness of things himself.