An interesting article about how to think about designing Artificial Intelligence for playing poker. It's high level but gives you a good feel about how this writer went about the design. You also learn the basics of Poker at the same time:
"It is very important in this type of project, for any AI project really, to fully understand the rules of the game and the tactics typically used to win. So when I worked on poker I spent a fair amount of time reading the best books and playing the game, both against existing AI and real humans of various levels of experience." (from Poker AI: A Starting Point)
And here's a follow up article on the same topic: "Aces High: Numerical Techniques in Poker AI"