Link
The ESRB has changed the rating from Teen to Mature citing the reason as
"The content causing the ESRB to change the rating involves more detailed depictions of blood and gore than were considered in the original rating, as well as the presence of a locked-out art file or 'skin' that, if accessed through a third party modification to the PC version of the game, allows the user to play with topless versions of female characters,"
not that it's really going to change anything, but it's a bit silly of Bethesda to leave the nude skin in the original game files, especially after the GTA Hot Coffee thing.