Dr. David Ho doesn't look like a gambler. With his boyish face and slender build, he could more easily pass for a teenager than for a 44-year-old father of three--or, for that matter, for a world-renowned scientist. In fact, when he was an undergraduate at the California Institute of Technology back in the 1970s, Ho hung around the blackjack tables in Las Vegas, tilting the odds in his favor by memorizing each card as it was played. He got so good at counting cards that he was thrown out of several casinos.