Nudist Tips 

A woman standing on a street corner breast feeding her child may be awarded more leniency by the law than a woman stripping for money on the opposite corner. Many states and commonwealths within the United States don't have laws specifically prohibiting public nudity. If an arrest is made of an individual or individuals who are nude in public it's usually under misdemeanor nuisance laws such as disturbing the peace, impeding a public right-of-way, things like that.

But is nudity freedom of speech and thus protected by the First Amendment?

If the nudity is the only way to express a certain concept or idea or viewpoint, yes, most likely it would be considered protected speech under the First Amendment. Or if the person "speaking" by nudity had no other means of expression, yes, most likely it would be considered protected speech under the First Amendment. Factors such as the location where the nudity took place, who witnessed the nudity (those who expected to see such a display or those who would have likely not expected such a display in the given setting) would come into play as well in a court decision, but likely a case could be made for the viewpoint expressed or the necessity of the method used, that is, being nude to make one's point.