Today (June 23, 2010) would have been

Alan Turing's 98th birthday---if he had not died in 1954, at the age of 41.
I never met Alan Turing; he died five years before I was born. But somehow I feel I know him well---not least because many of my own intellectual interests have had an almost eerie parallel with his.
And by a strange coincidence,

*Mathematica*'s "birthday" (June 23, 1988) is aligned with Turing's---so that today is also the celebration of

*Mathematica*'s 22nd birthday.
I think I first heard about Alan Turing when I was about eleven years old, right around the time I saw my first computer. Through a friend of my parents, I had gotten to know a rather eccentric old classics professor, who, knowing my interest in science, mentioned to me this "bright young chap named Turing" whom he had known during the Second World War.
One of the classics professor's eccentricities was that whenever the word "ultra" came up in a Latin text, he would repeat it over and over again, and make comments about remembering it. At the time, I didn't think much of it---though I did remember it. Only years later did I realize that "Ultra" was the codename for the British cryptanalysis effort at Bletchley Park during the war. In a very British way, the classics professor wanted to tell me something about it, without breaking any secrets. And presumably it was at Bletchley Park that he had met Alan Turing.
A few years later, I heard scattered mentions of Alan Turing in various British academic circles. I heard that he had done mysterious but important work in breaking German codes during the war. And I heard it claimed that after the war, he had been killed by British Intelligence. At the time, at least some of the British wartime cryptography effort was still secret, including Turing's role in it. I wondered why. So I asked around, and started hearing that perhaps Turing had invented codes that were still being used.
I'm not sure where I next encountered Alan Turing. Probably it was when I decided to learn all I could about computer science---and saw all sorts of mentions of "

Turing machines”. But I have a distinct memory from around 1979 of going to the library, and finding a little book about Alan Turing written by his mother, Sara Turing.
And gradually I built up quite a picture of Alan Turing and his work. And over the 30 years that have followed, I have kept on running into Alan Turing, often in unexpected places.