Mojang hired an AI specialist some time back to open the door to smarter villagers and monsters. The monsters part seem to be going well, as the guy is now going through the mobs one by one to improve on their behavior.
Zombies are already done, and will probably be included in the update to 1.2. Instead of rushing blindly at you, they now do some pathfinding to go around obstacles and walls. Hiding behind a simple pillar won't be enough anymore! Skeletons are currently being worked on, and they'll be avoiding sunlight as much as possible. This means that some will survive the nights, hiding under trees for the best opportunity to land a hit.
I fear to see what they'll do with the Creepers...
Topic: Fear for your lives!, Smarter mobs on the way!