It's widely known to atheists, simply because many of the old laws seem outlandish to us...however, for Christians, they are fairly insignificant portions of the Bible. And it is not as if they haven't read them, they just don't see them as particularly memorable as we do.
Frankly, concerning the shellfish, I don't think it's a big deal. So what if a God told you what you could or could not eat? It's entirely reasonable that in those days the "unclean" animals SHOULD have been avoided, simply because they were likely to cause sickness. People who make a big deal about those "silly" laws are incorrectly equating our culture to that of many generations ago. Of course they seem silly to us now, but at the time, there may have been good reason for it...
Now, don't get me wrong, I obviously don't think the food laws actually came from God, but from the men writing the Bible who were speaking out against things that had been observe to be harmful to their culture.
There is a limit to my old law acquiescence, however. Things which I think are obviously immoral, such as slavery, will get no defense from me. Yes, it was a part of their culture...doesn't make it ok.