UPDATE: At the 26 minute mark of the below Youtube, Leslie Mann confirms that she did nudity for the film.
http://youtu.be/HBHvRj2rR7s?t=26m
Bartap from the Celebtop forums saw "This is 40" yesterday and reported that Leslie Mann hase two topless scenes.
"I saw a screening of This is Forty today and I am pleasantly surprised to report that Leslie Mann shows bare breast in two different scenes, once during a mammogram, and once later on when she's trying to seduce Paul Rudd. Megan Fox also spends a lot of screen time in lingerie or a bikini (no thong, sadly)."
Of course, during Mann's previous "nude" scene in "The Change-Up", she wore prosthetics. When asked about this, here is Bartap's response.:
"I don't claim to be an expert, but 1) it looked real to me and 2) Judd Apatow has done nudity in his movies before (and I don't think he's ever resorted to CGI)."
We will probably have to wait until the movie comes out in theatres on December 21 to be sure.