UPDATE: Tonight, I just received a very detailed review from a trustworthy source, detailing all of Scarlett Johansson's nude scenes in Under The Skin. This is the same source who gave me information about Sarah Gadon's nude scene in Enemy (which like Under the Skin, is also showing at the Toronto International Film Festival).
Basically, Scarlett shows full nudity front and rear, but some of it is from a distance, and most of it is dimly lit.
Here, in verbatim, is the review he sent me.
Scarlett has four nude scenes in the movie and countless others where she's in her underwear or showing off major cleavage.
First scene is just a few minutes in. Johansson is completely nude and stripping an unconcious woman of her clothing. Butt, boobs, and brief full frontal. Pretty lengthy scene. You see her head to toe all in one shot, but it's back lit at distance. It's not as clear as it could be, but it's all still there.
Second scene: All black void as her character seduces a severely deformed man. Starts off in her underwear and takes it off as she's backing away from the guy. Full frontal from the side at a distance. Best lit of all the scenes.
Third scene (and by far the best): Her character is examining her body in front of a mirror for a good few minutes. Numerous angles, front and back. Boobs, butt, and bush. Dimly lit.
Fourth scene: You see her butt when she gets up out of bed after having sex with a guy. Dimly lit but a good view.
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There are a number of reports, stating that Scarlett Johansson has nude scenes, including some full-frontal) in Under The Skin, which has screened at Telluride and just today has screened at the Venice International Film Festival. The question is how much can actually be seen.
Here is a Hollywood Reporter review mentioning the full-frontal nudity:
"Johansson herself is not the source of the film's problems. She totally commits to the part, to the extent that she even performs several scenes of full-frontal nudity. Unfortunately, many, if not most, of the people who ultimately will check out this film will probably do so solely in order to check out those scenes."
http://www.hollywoodreporter.com/race/telluride-under-skin-starring-scarlett-618843
However, the blogger behind the Batty for Nudity blog contacted another reviewer of the film, and he mentioned that the view is not clear.
On the other hand, some brief mentions of the scenes from people elsewhere makes it sound like quite a bit is actually visible. This film is coming to the Toronto International Film Festival next week so hopefully we will definitely know by then what can actually be seen.