The ESRB has changed the rating of the PC & Xbox 360 versions of Oblivion from T (Teen) to M (Mature). The ratings board enacted the change because the game contains stronger depictions of violence than what were featured in the content that was submitted to determine the original rating.
But more importantly, the new M-rating is a result of hidden (archived) topless skin files that can be accessed in the PC version via a fan-created mod.