1up have previewed S.T.A.L.K.E.R - Shadow of Chernobyl and here's a snibblet:<blockquote>The key to this is a unique life-simulation system that drives every creature in the game. The best example of this comes in the form of the other treasure hunters prowling the badlands. These aren't just traditional scripted A.I. enemies. These stalkers actively compete with you, trying to beat you to the artifacts and technology you pursue, reacting to you based on your previous behavior and reputation, and setting their own objectives with the same degree of freedom that you enjoy. You can approach them as hostiles, or you can cooperate with them and even trade goods, weapons, and equipment. Motivated by greed, cowardice, perceived advantages, and more, and occasionally operating in clans, stalkers will be the deadliest adversaries in the game.</blockquote>Link: Stalker Preview@1up