Growing up, my mom had this little statue among the Christmas decorations of Santa bowing before a baby Jesus in the manger. I remember asking her one year why she had it, and she said it was a gift and didn't want to be rude because the gifter often visited our house. But, the next year, Santa didn't come back out of the attic. I think that was the last straw in pretending we bought into the whole "Santa thing" in my growing-up house.
I never believed in Santa Claus, at all. I don't think it's about me not remembering. I think I had two older sisters who would have been happy to relay the truth to me, but I also think I had parents who weren't really into Santa either. My family always had enough "magic" of our own, believe me!
When I was in fourth or fifth grade, my oldest sister started signing her stocking gifts to me "Love, Rudolph," but it wasn't like she expected me to believe that they were from him. We just enjoy Christmas music and lights and movies and cookies and I guess the whole "Santa" thing is a part of that.. so here's a cassette tape and a Lip Smackers from a Reindeer!
As a Christian, I am neither adamantly for or adamantly against Santa. My personal thoughts are as follows:
I want to teach my children to be grateful for things, so why give them a heap of presents and then not ask them to thank anyone? Like me and their father, for example, who picked them out, paid for them, and wrapped them.
I firmly do not want my children to ruin the fun of other families (to the best of their ability). So my kids have been warned: Santa isn't real, but it's ok that other people want to pretend he is. You may not spoil it for people who are enjoying that make-believe.
I want to teach my children the most important truth of Christmas. There are already so many, many things competing for my children's understanding of truth (Star Wars, for example). The last thing I want them to wonder about is if the story of Jesus coming to earth because of his overwhelming love for them is true. So to me, rather than saying "this is all true!" and then later telling them "well, that one part was just pretend," I'd rather just be up-front right away.
Personally, I don't think there's any valid argument that Christian families can't "do" Santa; we just don't. Your kid's eternal fate will not be affected by whether or not you remembered to drink the room temperature milk and eat the stale cookies quickly at 5:43 am before they run downstairs. I think the things that will impact their perceptions of Christ and Christmas are far more about your pouring the Gospel of grace into them throughout the entire year.