I had my suspicions about Santa pretty early--as a too-curious preschooler snooping in my parents' bedroom, I found boxes for some gifts that had been "from Santa"; my mother had made up some story explaining it. Later (6 or 7 years old?) I found a page stuffed into a drawer that had been ripped out of a book--it explained how to tell your kids that Santa wasn't real. I read all of the books on the shelf at home, including the parenting book; that was the bit of knowledge my parents wanted to hide from me! (I suppose they thought I would be too young to understand some of the other stuff I'd find, but that I would understand that if I found it. My parents really had no idea how to deal with a young voracious reader.)
So I knew that Santa wasn't real but that my parents (my mother, really) cared that I not find out. I don't think Santa in particular affected me much in part because I was a voracious reader--I knew a lot of things that were different than what my parents told me, and I also knew that most parents were advised that kids might not be ready to know them. (Like I said, I read their parenting book.)
Knowing that I couldn't trust my parents to tell me the truth about a lot of things (not just Santa) because they thought it better that I know a pleasant lie, and also that they really had no idea what I was and wasn't ready to hear, had a tremendous effect on me. I didn't trust them even when I should have, in fact; I rarely trusted people to be telling me the whole truth or to have good judgment about what I should and shouldn't be doing. (I also grew up in a weird household, the main thing being that my mother was hospitalized for mental illness when I was 11.) It was good for me in some ways--but there are some big things I should have sought someone's guidance for, if not my parents', but I simply had no idea how to go about it or even that I should.
I found a page stuffed into a drawer that had been ripped out of a book--it explained how to tell your kids that Santa wasn't real.
Shades of what happened when I found the secret hidden volume of my beloved Childcraft set, the "Guide to Parents"! Only in my case, that's how I found out what teenagers were supposed to be like and that's when I decided never to go there.
I've long entertained a dubious regard for the practice of lying to children about the existence of Santa Claus. Parents might claim that it serves to make children's lives more magical and exciting, but as a general rule, children are adequately equipped to create fantasies of their own without their parents' intervention. The two reasons I suspect rest at the bottom line are adherence to tradition, and finding it cute to see one's children believing ridiculous things.
Personally, I considered this to be a rather indecent way to treat one's own children, and have sometimes wondered whether a large proportion of conspiracy theorists owe their origins to the realization that practically all the adults in the country really are conspiring to deceive children for no tangible benefit. However, since I began frequenting this site, I've been exposed to the alternate viewpoint that this realization may be good for developing rationalists, because it provides children with the experience of discovering that they hold beliefs which are wrong and absurd, and that they must reject them.
So, how did the Santa deception affect you personally? How do you think your life might have been different without it? If your parents didn't do it to you, what are your impressions on the experience of not being lied to when most other children are?
Also, I promise to upvote anyone who links to an easy to register for community of conspiracy theorists where they would not be averse to being asked the same question.