Princess' or princess's

After using the spelling of princess' for years, I was just told recently that princess's is the correct possessive form.

Apparently, I am not the only one feeling confused about having an S behind the apostrophe.

http://osdir.com/ml/education.english.engfor/2006-03/msg00031.html
But there are two major systems for how you would write it out:> > 1. The princess's golden hair.> 2. The princess' golden hair.> > In the United States, newspapers tend to use system 2, and academic and> book publishers tend to use system 1.

http://wiki.answers.com/Q/What_is_the_correct_spelling_of_the_possessive_form_of_princess
There are two accepted forms for possessive singular nouns ending in s: Add an apostrophe (') after the existing s at the end of the word: princess' Add an apostrophe s ('s) after the existing s at the end of the word: princess's Examples: The princess' nanny took her to the museum. The princess's nanny took her to the museum.

http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv57.shtml
When I was in school and taught about apostrophes, one thing I remember was that nouns ending in a double 's' simply took the apostrophe without adding another 's'. Examples: the princess' crown and the boss' secretary. In the USA you would see princess's and boss's as standard practice. Has it come to this in the UK?

Gotta say though that the majority of information I found online rooted for Princess's as opposed to Princess'.