It seems that all of the answers have gravitated towards depictions of sex. Either because nudity equals sex (while in Hollywood, this appears to generally be the case - it's extremely rare to see a nude scene that isn't purely for titillation, rather than because someone just woke up or got out of the shower), or because it hadn't occurred to them that they might be separate.
If you want to know if social nudity - namely nudity that is purely nonsexual - has any effect on children, just ask a nudist.
This document explains one nudist club's stance on the effects of social nudity on children, and cites several studies. The conclusion they reach is that there is absolutely no negative effect, and there may be a positive effect on the psyche. The basic nudist philosophy is that the extreme modesty of the Victorian era was in fact harmful (which was proven by many studies before and since), and as such, perhaps its inverse - a total lack of modesty - is beneficial.
Studies aside, nudists themselves have observed no obvious negative effects in children - very young children especially like to be naked, and generally the idea that there's nothing inherently wrong with the human body in its natural state promotes better body image in older children as well. See the bottom of the document above about Casler's study, and the older children's reactions in his interviews.
It's generally parents' negative reactions to nudity in TV and film that are most harmful to children. Especially if that reaction is particularly unhinged and panicked - see also:
Super Bowl XXXVIII halftime show controversy
Edit:
There was a paper written on this subject (actually, of children witnessing parental nudity and sexual activity, which of course, is different from what you see on TV, since parents have such a central and authoritative role in a child's life) in the Journal of Sex Research. The paper can be found on here.
Consistent with the
cross-sectional retrospective literature (and with our expectations), no harmful main
effects of these experiences were found at age 17-18. Indeed, trends in the data that were
significant at p [less than] 0.05 but did not reach significance following the Bonferonni
correction indicated primarily beneficial correlates of both of these variables. Exposure to
parental nudity was associated with positive, rather than negative, sexual experiences in
adolescence, but with reduced sexual experience overall. Boys exposed to parental nudity
were less likely to have engaged in theft in adolescence or to have used various
psychedelic drugs and marijuana.
Taken as a whole then, effects are few, but generally beneficial in nature. Thus, results of
this study add weight to the views of those who have opposed alarmist characterizations
of childhood exposure both to nudity and incidental scenes of parental sexuality.
This is very likely the kind of thing (historically speaking, of course, which the paper touches on) that the MPAA and the government bases such things upon. Mostly this seems to harken back to Freud and early students thereof, who assumed that nearly every psychological disorder originated from witnessing such things as a small child. The paper linked above questions this assumption and tries to find actual clinical research to support the claim.
