Her title is NOT Princess. Shes the DUCHESS of Cambridge. (Just like Fergie wasnt Princess Sarah, she was the Duchess of York).
What are the rules for the title for the wife of a prince? Was Diana also a duchess rather than a princess? Was "Princess Diana" incorrect? It seems to me that was commonly used when referring to her.
When William becomes first in line to the throne (after his father, Prince Charles, presumably becomes King), Kate will become Princess Catherine.
Upon marriage, Diana became the Princess of Wales. From Wiki "Though Diana, Princess of Wales was commonly called Princess Diana after her marriage to Charles, Prince of Wales, it was officially incorrect, as Diana herself pointed out, because she was not a princess in her own right. Similarly Camilla, Duchess of Cornwall, is neither Princess Camilla nor Duchess Camilla."
“Diana, Princess of Wales” or “The Princess of Wales” was the correct form