How would you address Kate Middleton now that she's princess?

Answer 1

She isn't a princess. She is Her Royal Highness the Duchess of Cambridge. (It is likely, that at some stage Letters Patent will be issued making her a princess. Her husband, a Prince from birth, bears that rank still, so that he is Prince William, Duke of Cambridge.)

In conversation, or in correspondence, the correct form of address is 'Your Royal Highness' at the outset of the conversation/correspondence, subsequently 'Ma'am' (to rhyme with 'ham', not 'marm'). But, if you were presented to her, you would merely shake hands. It would be for her to decide whether or not to engage you in conversation. One of the biggest violations of Royal protocol would be to initiate conversation with a member of the Royal family. The same applies to touching them (apart from the handshake). Richard Branson found this to his cost. He had invited Princess Diana to inaugurate Virgin Atlantic, and, ever the showman, tried to put his arm round her for a photo-op. He was definitely and firmly given the cold-shoulder by the Princess who rapidly disengaged herself!