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:

Diablo III

Platform: Macintosh, Windows PC

Rating: Mature

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)

Succubi? Not a surprise they might return to Diablo III, but we’ve never seen any concept art or heard any mention of them prior to this. So… will they be hot, like in D1? Or hideous, like in D2?

The game is yet to be rated in Europe.