A warning page alone won't save you.

I think what the attorneys and others who suggest them are saying is to take real, meaningful steps to show that you're trying to keep porn away from those (such as Aunt Thelma) who don't want to see it.

I look at it like this. In the worst case, if I'm put in front of a jury of Aunt Thelma and Grandma Smith and Uncle Elmo and the like, I want to be able to stand in front of them and credibly say that we took all the reasonable steps we could - warning page, tags to make filtering software work, encoding all our images with the invisible adult content watermark, etc - to keep people who don't want to or shouldn't see our content away from it.