Artificial Intelligence in online game using Google App Engine
Will your game be turn based or real time?
Either way, I think you have 2 options to look into.  One is to use the Cron feature so you can schedule NPC updates at regular intervals, the other is to stick a "update NPCs" task into the Task Queue every time a human player moves.
I am currently in the planning stages of a game for google app engine, but cannot wrap my head around how I am going to handle AI.  I intend to have persistant NPCs that will move about the map, but short of writing a program that generates the same XML requests I use to control player actions, than run it on another server I am stuck on how to do it.  I have looked at the Task Queue feature, but due to long running processes not being an option on the App engine, I am a little stuck. 
I intend to run multiple server instances with 200+ persistant NPC entities that I will need to update.  Most action is slowly roaming around based on player movements/concentrations, and attacking close range players...(you can probably guess the type of game im developing)