Kate Middleton and Prince William's title has been told.
When the baby is born it will be called HRH Prince or Princess (name) of Cambridge.
A source has said:
“The baby will be called HRH Prince or Princess (name) of Cambridge. In the same way as HRH Prince William of Wales, and HRH Princess Beatrice of York. Please note, there is no definite article (no “the”) — the child will not be The Prince or Princess of Cambridge. Incidentally, Cambridge is a city.”