According to an announcement made by the ESRB this afternoon, the change in rating is due to what it says is "previously undisclosed content" discovered through "post-release monitoring and play-testing activities." It looks like after spending more time with Oblivion the ratings board decided that the game is more violent than they were led to believe. The announcement says that this content was "not fully disclosed when the game was originally submitted to the ESRB for rating."
Another reason for changing the rating from T to M is the existence of a locked-out art file on the PC version which includes graphics for topless female characters. This content is only unlockable by using a third-party mod, but the fact that the content itself exists on the DVD is enough to warrant the change. If any of this sounds familiar to you, it's because Grand Theft Auto: San Andreas went through a similar ordeal with the Hot Coffee mod.
I don't mind you swinging that sword at me, just keep those breasts covered.
Bethesda Softworks is already taking measures to update the ratings label on all unsold copies of Oblivion, and future shipments of the game will have the new rating clearly labeled on the packaging. The PC version of the game will include a new "nudity" descriptor on its rating label. Future copies of Oblivion will ship with the offending files removed and will not have the "nudity" descriptor. There is also a patch on the way which disables access to the topless skin file on the PC. A second announcement from the ESRB concludes with a paragraph about how mods for PC games are user-created programs that are not considered when assigning a game a rating and factoring how they can affect the content of game.
Not long after these announcements were made, the Interactive Entertainment Merchants Association released an official statement from their President, Hal Halpin, praising the speed at which IEMA retailers reacted to the new rating. Halpin says that the ESRB's decision "stands as a testament to the effectiveness and commitment to the industry's self-regulatory efforts" and adds, "The effective change in sales policy was immediate." There are already reports of some retailers pulling copies of Oblivion off their shelves until they can be labeled with the new M rating.
