We have a very non-religious house - a bit of Jewish, a bit of Christian, mostly a whole lot of apathy. I told my kid from the get-go that there was no Santa. His response? "Yes, there is, I saw him at your party at work." But we don't buy him any Santa presents.
I remember finding out at around 5 or 6 from some kid at school. It did not in any way blow my mind.
I signed up this morning too! I've been toying with the idea for years but this is the year it's going to happen! Maybe!