This article at Time magazine interview the guy who wrote a program that beat human beings at poker. He explains how it happened. The short answer is that computers can do a lot of math really fast, and he designed the program to do the kind of math that helps win at poker.
Also, since a lot of poker playing is reading one's opponent as closely as one reads the cards, a computer has the advantage of not too many giveaway nervous tics.
This is not as big a deal, I think, as some people might like it to be. After all, the computer beat people at poker. But who designed the computer? It's like the video of the orangutan who figures out how to use a saw to cut a tree limb. That's pretty cool, all right, but not as cool as being the species that designed the saw. I don't figure on waking up to either Terminator or Planet of the Apes tomorrow morning.