Be you a law geek, a grammar geek, or both, or neither, I recommend to you this article, addressing the Supreme Court's use of the additional "s" when constructing the possessive of a singular noun that ends in s. It contains such brilliant passages as this:
Whereas Thomas apparently believes that whenever a singular noun ends in s, an additional s should never be placed after the apostrophe, Souter has made equally clear his conviction that an s should always be added after the apostrophe when forming a singular possessive, regardless of whether the nonpossessive form already ends in s. With this acrimonious undercurrent simmering in the background, Souter boldly began his Marsh dissent as follows: “Kansas’s capital sentencing statute provides . . .” This dramatic and gratuitous use of the possessive was an obvious attack on Thomas, who, as one of three s-ending members of the Court, is viewed as a role model for the millions of children who grow up with the stigma of grammatical ambiguity attached to their names.
The rest of the article takes on the matter with comparable vigour. For the record, apparently the Court's view, by a 7-2 majority, is that the extra "s" should be omitted. This is my view as well. According to the article this approach, while popular, is technically incorrect.