“Diana, Princess of Wales” or “The Princess of Wales” was the correct form
While I thought it was, that brings up other questions:
Why are some prince's wives (such as Diana) titled Princess and other princes' wives (such as Fergie) titled Duchess?
Does the territory of which one is a prince determine that?
Will the older of Charles' sons (sorry, I forget which one is older) become The Prince of Wales (and his wife The Princess of Wales) on the death of Prince Charles?
Will (and Kate) are one step lower than Charles and Di. Will is the "heir apparent to the throne". After Queen Liz passes on, Charles will become King and Will will become "heir to the British throne". He will also get the title Prince of Wales (the title KING trumps all others)and Kate will become Catherine, Princess of Wales. If they have kids, their first born son will become "heir apparent to the throne". He will not be the Prince of Wales until King Charles passes on.
I met a gal who thought Diana was Princess of Wales because she came from Wales...Itried in vein to explain it all to her but she never did get it.