There was the Supreme Court case of Reynolds vs. the United States, where polygamy, incest, and neglect of children were all ruled illegal regardless of marriage license or state sanctioning of the actions, and they specifically defined the ruling as a means of keeping a common rule for everyone to follow. Only problem is that nowadays, people speak so loudly against the idea of common law, when having a common law actually makes society somewhat decent.
Very interesting — the Supreme Court relied on common law for condemnation of polygamy, etc.! Fascinating.
Common law, as you point out, is a thin reed indeed. What, after all, is common law in San Francisco?