Gamespot has details on Bethesda's response to the ESRB
for their (some would argue) knee-jerk reaction to fan-added elements of Oblivion. From the article: "There is no nudity in Oblivion without a third party modification. In the PC version of the game only - this doesn't apply to the Xbox 360 version - some modders have used a third party tool to hack into and modify an art archive file to make it possible to create a mesh for a partially nude (topless) female that they add into the game. Bethesda didn't create a game with nudity and does not intend that nudity appear in Oblivion."
They go on to state they submitted a 60-page document detailing the violence in the game. If anyone is at fault here, I think it's the ESRB.