As in the British peerage, the wife's title is taken from her husband's, If William is created a duke, Kate will most certainly carry the title of Duchess. And.. once they do say "I do", Kate most certainly be an HRH and as royal as they come.
The people that would refer to Diana, Princess of Wales as Princess Diana were most certainly wrong as she has never been a Princess in her own right. She was known to correct folks on this matter herself when addressed as Princess Diana. After her marriage to Charles, she was The Princess of Wales taking his titles/styles. After the divorce, she only used the divorced style Diana, Princess of Wales.