The Entertainment Software Ratings Board, a trade organization that reviews video game content, announced yesterday that it had discovered "more detailed and intense depictions of blood and gore" than it saw in its pre-release review.More details over here.
The board also objected to a software modification that was circulating among gamers on the Internet that allowed players to create topless versions of female characters in the game.
Bethesda Softworks said in a statement last night that it would place "M" rating stickers on games already in stores but that the game would not be recalled. While agreeing to comply with the board's decision, the company disputed the rationale behind the higher rating and said it would not alter the game's content to keep a "teen" rating.
Elder Scrolls: Oblivion game rating changed to Mature

Elder Scrolls IV: Oblivion is one of the best selling games of the moment. However, it has now been relabeled with a mature rating, prohibiting sales to children under age 17: