Bethesda's report did not list nudity because "[there] is no nudity in Oblivion without a third party modification." The existence of topless heroines is not a result of unlockable content, but rather, a result of third party tools used to modify the game's art archive. With that said, it's difficult to justify the ESRB's decision to include "nudity" as part of the PC version's explicit content.
Bethesda also confirms that there will not be a recall, nor will the game's content be changed in order to retain the Teen rating. Instead, future copies will be printed with Mature rating labels, and M-rated stickers will be applied to all existing unsold copies of Oblivion.
Note: Both the PC and Xbox 360 versions of Oblivion have been re-rated.