I don't think the nvde mods (not files; there were no nvde FILES in the vanilla game to my knowledge - people may be confusing this with GTA's Hot Coffee incident) were the main reason the rating was changed. My recollection is that the ESRB ratings board didn't discover the Dark Brotherhood quest's more gorey bits until well after the game released, and then changed the rating to M retroactively as a result. They did include mention of nudity mods in their notes, but I don't believe it was the primary reason.

An AO rating is out of the question, as it would compromise sales too much.

http://www.esrb.org/about/news/downloads/oblivion_release_5.3.06.pdf

Yes, they found alot more gorey things in the game but a nvde art file I would believe would be the straw that broke the camels back.

I don't really know what the rating categories mentioned on here mean, but in the UK Oblivion was a 15, so I could see Skyrim being the same. Although, seeing as how it's always being mentioned how much more "brutal" this game is supposed to feel, maybe it will go up to 18.
...
Just had a quick check, and Morrowind was a 12, so I could see it taking another step up.

M duurrrrrr, they said they wanted to make killing enjoyable lol

I don't want violence toned down but I don't want the "Mature" rating to be wasted on six scenes like those of Fable. Those were pathetic and pointless. This is a video game.

I really can't see how it won't be 'M' :shrug:
Honestly, I just want them to make the game, the way they want to make it, with no consideration to the rating. Let the rating land where it will, without the need to 'keep-out' or 'dumb-down' any features to make the game fit within someones preconceived notion of what a game should be rated.

Hope its rated MA15+, with nice but not overdone gore, skooma dens etc