It will have an M rating. While lower ratings are better for sales in general, it is important to note that having an AO rating is signing your own death warrant. No AO games are allowed on consoles, not even Microsoft or Sony want AO games to taint their sales. I think Oblivion, which was forced into its M rating when it would rather be T, proved to Bethesda that M rated games still sell. And certainly Fallout 3 continued proving that as well, which embraced its M rating and was one of the best games that year. Going back to Elder Scrolls, I think they know how far they want to push it, and they'll go there for as long as it pleases their consumers.
Thematically even, Skyrim should be M. Oblivion was M, and Skyrim is supposed to be much more brutal and unforgiving than the dainty imperial province of Cyrodiil. It's only natural.
The problem with that is I have the original PC release of Oblivion and there is no such locked out file. I'm speaking from experience, not from 3rd hand sources.
I have the original game too (My disc still says T on it) and I do remember seeing that art file in its early days. You can't find it now, however. It was patched out as soon as it became an issue, and none of the later M rated bundles came with it included, such as the GOTY edition. It was still a factor in the increased rating of the game, however, and I think it was not in Bethesda's intention to have that change the rating of their game from T to M. They would have been very satisfied with a T rating at the time.
On the bright side, it made history. Only a small handful of games have ever had their ratings changed post-release