Artificial Intelligence in online game using Google App Engine
If the game is turn based then it would probably be best to avoid the Cron task and just update the NPCs every time the player moves. I'm not sure how big of a map you are planning on but you may consider even having the player object find the NPCs that are close to it and call their AI routine.  That way NPCs that are out of range of a player wouldn't move at all which may save on resources.  Not sure if that matter though.
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)