The US game rating board, the ESRB, has posted their official rating for Diablo III, and as expected the game is rated M for Mature. The full info from the ESRB website:
Platform: Macintosh, Windows PC
Content descriptors: Blood and Gore, Violence
Rating summary: This is an action role-playing game in which players assume control of a mortal hero (e.g., witch doctor, barbarian, wizard) who must defend humanity from a demonic invasion. From a 3/4–overhead perspective, players traverse dungeons and use swords, axes, and magic attacks to kill a variety of human-like enemies (e.g., zombies, demons, succubi). Battles are accompanied by slashing and flesh-impact sounds, screams of pain, and frequent blood-splatter effects; creatures often explode into bloody fragments as multiple enemies are dispatched at once. Some levels depict burning corpses and dead villagers amid large pools of blood.
Online Notice: * Includes online features that may expose players to unrated user-generated content (Macintosh, Windows PC)
The game is yet to be rated in Europe.