I don't have time to play computer games very often, and in truth I've yet to play any of the Halo games. Nevertheless, I found this list of 42 AI tricks to assist your game from the game team at Bungie responsible for Halo 3 very interesting. Each of the shortcuts the article lists could be the starting point for an agent research project: how would we really like an autonomous system to do that? Another interesting aspect was the predictability of the behaviours: in some cases, the AI system was de-tuned or simplified in order that the players would have more chance of predicting what the AI characters were going to do, both to make the game more playable and to increase the sense of realism. Interesting stuff.
Kudos to Bourbaki in #ai on freenode.irc for the link.