Okay, I've played through a few times with some shop runners, and pored over boardgamegeek (http://boardgamegeek.com/thread/397178/rules-question-locked-and-hidden-doors) -- Here's my best interpretation.
Doors- 1 movement cost
Fundamentally identital to a passageway. The door itself plays an important role with monsters. If a monster is tiny or human sized, they can usually open doors, unless directed otherwise with their monster card. Huge monsters generally can't go through doors unless they have a skill dictated on the card.
Locked Doors- 3 movement cost
Thieves can treat this as a normal door, meaning it will only cost 1 movement point to pass through. Monsters generally can't pass through these unless their cards specify they can break a door down-- allowing all monsters also moving during that turn to pass through the broken doorway. During combat, the movement cost is ignored, and the room is considered "adjacent" and a neighboring munchkin can participate in combat unless the room tile states otherwise. In the rulebook, this is justified by the statement that combat movement "just happens," and doesn't cost anything.
Hidden Doors- 3 movement cost
Wizards can treat this as a normal door, meaning it will only cost 1 movement point to pass through. Nearly all monsters cannot pass through this door, even if they have the ability to break down doors. Only monsters which can see hidden doors, Like King Tut, can pass through a hidden door. Hidden doors do not have to be "discovered", and are not treated as a wall by munchkins. All combat rules for locked doors apply to hidden doors as well.
At no point do doors get treated as corridors unless a monster breaks it down. The presence of a door has the most significant impact on monster movement and movement cost, not munchkin movement.