Diana could actually have been addressed as the Princess Charles, same as Princess Michael of Kent is addressed by her husband's name. It's the same system whereby a married woman is officially eg Mrs John Smith not Mrs Joan Smith. She is only Mrs Joan Smith if she is divorced or widowed. If she then remarried, she would become Mrs Dave Newhusband.
The Duchess of Cambridge also has the title The Princess William, but the Duchess is a more user-friendly title for the masses who have long ago given up addressing married women as if their first names have disappeared. Nobody minded about Princess Michael losing her first name as the British masses didn't really care about her or her husband (just another extraneous royal), plus she was a foreigner, whereas everyone was mad for Lady Di and wouldn't have stood for calling her Princess Charles.
Royal Princes often get an extra Dukedom when they get married - wedding present from Brenda - such as Randy Andy becoming the Duke of York. When Kate becomes Queen she will of course be Queen Catherine, just as Diana would have officially got her own name back.
Camilla is officially the Princess Charles now, but the royals wisely backed off that. She will also be Queen Camilla when Brian accedes to the throne, but they'll probably stick to calling her the Duchess of Cornwall because of her Past.
I must be bored.
