According to Yahoo! News, a Wisconsin court has held that a person who is voluntarily naked in front of others still has a reasonable expecation not to be videotaped. (See: Wis. court: Nude people still have privacy rights - Yahoo! News.) Unusual case but makes sense to me.
Via Photo Attorney.
Added: I've read the decision and have a bit more information (enough to make me change the title of this blog-post to be a bit more clear.
In this case, the defendant was charged with violating a Wisconsin law that criminalizes recording someone in the nude when they have a reasonable expectation of privacy. The complainant was voluntarily disrobed in front of her then-boyfriend, who had hidden a camera under some laundry. He argued that she had no expecation of privacy as she knew he was able to see her disrobed. The court took a much more context-specific reading of what is a reasonable expecation of privacy.