Princess is a title usually conveyed by birth, rather than marriage. As Prince William's wife she's most formally refered to as Princess William, failing that she's the Duchess of Cambridge or Countess of Strathearn in Scotland.
Always had my suspicions about him.
That's a load of bollocks. Princess Diana was never called Princess Charles.
No it is quite true. Her formal title is Princess William, Duke of....
Princess Diana was both never an actual princess, despite what people called her and was from marriage the Princess of Wales, which brings a bit more clout than Duchess of Cambridge.
I believe the Queen has an aunt who is called Princess Michael--she married Prince Michael of somesuch and he never got a higher title.
When Mr. King gets married, his wife is Mrs. King
When the King gets married, his wife is the Queen
When Prince William gets married, his wife is Princess William
When Ms. Queen gets married, her husband keeps his name
When the Queen gets married, her husband gets no title