World of Warcraft Postal System & Realm Types

Blizzard has released some preliminary information about realm types and a postal system, two features set to debut shortly in the World of Warcraft beta. The section about realm types provides some good information about how PvP will work, so it's a highly recommended read. Here's a snip:
Standard Player vs. Environment Server

Players have the ability to attack almost all opposing faction NPCs including vendors and quest givers. If a player attacks an opposing faction NPC, players of the opposite faction are then able to target and attack that player. Whenever an NPC is attacked, players who share the same faction as that NPC will receive a system message indicating that a specific town is under attack.

Please be aware that some quest NPCs might not always be available. If they have been killed, players should wait for the NPC to respawn.