Development of robust AI is difficult work, but the guys at Epic have been at it for a long time. I was really impressed with the behavior of the AI in Unreal Tournament 3. After almost a decade of evolution and refinement, they have at last come up with bots that are every bit as irritating and idiotic as the players you meet online.
You can still tell the difference, though. If you look closely at all the players stealing your kills, driving away with your vehicles, and blocking your shots, you'll notice the human players are the ones complaining about team balance and calling you a fag.
Shamus Young is a programmer and writer by trade, videogame nitpicker by inclination. If you have the patience for more of his ramblings, they can be found at ShamusYoung.com.