Well, one thing to keep in mind is that most kids aren't taught about Santa because their parents are trying to set up a rationalist epiphany opportunity for them. Rather, they're taught about Santa because, well, the parents themselves were probably taught about Santa (and God, for that matter) when they were kids, and they probably just figure it's one of those things you do when you have children.
Plus (and I think there might have been an OB post about this once), many adults find ignorance/innocence of certain types in children to be "cute" or appealing in some way. I think the appeal of the Santa mythos for some parents is that it feels to them like they are giving their child a chance, if only a brief one, to live in a world where "magic" actually exists.
In any case, I got in trouble on multiple occasions growing up for talking about how the Easter Bunny wasn't real, how Minnie Mouse (at Disneyland) was a human in a suit, etc., in front of younger kids. That probably confused me more than anything else, more than the fact of having been told things that weren't true to begin with -- I felt like I was being pressured to perpetuate some weird group fantasy and had a terrible time figuring out what I'd supposedly done "wrong". I mean, you can still hunt for Easter eggs and exchange presents and have fun at a character-themed park with full knowledge that actual humans (and not supernatural entities or magical anthropomorphic animals) are behind the whole thing.
All that said, I don't know if it's possible to extrapolate out the role being taught Santamyths and Godlore as a child might play in someone's adolescent and adult rationality. I haven't looked to see whether a large scale survey has even been conducted, but I bet the results of such a survey would be interesting.
In my own limited sample set (consisting of myself and various people whose upbringing I know a little bit about), there doesn't seem to be a major correlation between the type/level of Santa mythos they were exposed to and how much they value truth, how much they appreciate/understand science, what their thoughts are on evolution (for example), etc. What seems to be a far more influential factor is whether the kid has opportunities to actually confront reality without necessarily being sheltered by privilege or convention in certain respects -- e.g., the more spoiled kids I went to elementary school with seemed totally uncurious about how things physically worked, what they were made of, etc.
So while I probably wouldn't personally tell my kids (and I don't have or want kids, but we're talking hypothetically here) that Santa was real -- I'd feel silly and fake if I did -- I don't think parents wanting to raise little rationalists need to dwell extensively on the Santa question so much as make sure their kids actually learn about things like cause and effect and basic physics and such through experience.
"I felt like I was being pressured to perpetuate some weird group fantasy and had a terrible time figuring out what I'd supposedly done "wrong""
Yes! A thousand times yes!
We stress the importance of distinguishing between fantasy and reality, but then people actively try to confuse their children as to what's real and what isn't - all the while thinking less of children because they're supposedly not good at it.
Related on OB: Lying to Kids The Third Alternative
My wife and I are planning to have kids, so of course we've been going through the usual sorts of debates regarding upbringing. We wondered briefly, will we raise our children as atheists? It's kindof a cruel experiment, as folks tend to use their own experiences to guide raising children, and both of us were raised Catholic. Nonetheless, it was fairly well settled after about 5 minutes of dialogue that atheist was the way to go.
Then we had the related discussion of whether to teach our children about Santa Claus. After hours of debate, we decided we'd both have to think on the question some more. It's still been an open question for years now.
Should we teach kids that Santa Claus exists? This isn't a new question, by any means. But it's now motivated by this thread about rationalist origin stories. Note that many of the posters mark the 'rationalist awakening' as the time they realized God doesn't exist. The shock that everybody, including their parents, were wrong and/or lying to them was enough to motivate them to pursue rationality and truth.
If those same children were never taught about God, Santa Claus, and other falsehoods, would they have become rationalists, or would they have contented themselves with playing better video games? If the child never realized there's no Santa Claus, would we have a reason to say, "You're growing up and I'm proud of you"?