I am still hooked on Halo, only 1.5 weeks later and I still can't wait to get home to play it. While browsing my email, I came across an interesting article (sent to a "cool stuff" distribution list I'm on) that takes about the AI of Halo 2.
http://stuffo.howstuffworks.com/halo2-ai.htm
The article goes on to explain how Halo 2 is different than most FPS (First Person Shooter) games to talk about having the player in a virtual world, as apposed to, like in Quake 3 for example, having the player be a camera and have the world flash by.
This enables the actual AI components to "think" and "react" to things that happen in their world. If you're into this stuff, the article is very interesting.