"The depiction of adults and children nude in the visual media has enjoyed constitutional protection in the United States since 1958, when the Supreme Court vacated a Court of Appeals finding that Sunshine & Health magazine could be obscene (Sunshine Book Co. v. Summerfield, Postmaster General, 355 U.S. 372). The right to depict adults and children in innocent nude poses has been upheld without a pause for 41 years. In case after case, the Supreme Court and lower courts have always upheld the constitutionality of "nudity without more," specifically referring to the nudist depiction as a fully constitutional form of expression."
http://nudism.name/legal.html