I read an interview with James Middleton today in which he referred to his sister as Catherine rather than Kate. Prince William also calls her Catherine in interviews.
I don't know why, but I find it mildly grating that since she got together with Prince William, her family and friends have suddenly begun referring to her by a different, longer version of her first name - especially when they presumably always called her Kate before, still do in private and everybody else refers to her as Kate anyway.
It just seems very contrived and a little pretentious or something I suppose. AIBU to wonder why Prince William and her family can't just call her Kate in public?