AI Director isn't so much about the actual artificial intelligence of the monsters (let's face it, zombies aren't supposed to be smart) as it is having the game itself know when, where, and how many zombies to start throwing at you from random locations.
HL and HL2 had some pretty decent AI, but we really do have a long way to go. A friend and I were working out a way you could do a "scent" based AI, but it would still rely heavily on the old waypoint system, so I think that's the major hurdle you'd have to get over before anything else. I think I read somewhere that L4D doesn't use waypoints, but I'm not certain (and the SDK still hasn't been released).
There were some guys working on a dedicated AI hardware chip, much in the same vein as those physics cards, but I don't know if anything actually happened with that (heck, did anything happen with those physics cards??).
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I'll brave the storm to come, for it surely looks like rain...