No more "Waity Katie," that's for sure! Kate has become more like Constant Catherine. According to protocol, the palace will now refer to her as "Miss Middleton," while the press and others are to use her full first name, Catherine (with a "C"--no clarification on why she has been Kate with a "K"). Catherine Elizabeth Middleton's married title will not be known until the day of the wedding, nor will Prince William's.
 
Although William was born a Prince, he has to be raised in rank to become a duke, a title that, traditionally, the queen chooses and then gives him as a gift on his wedding day. When the royal couple is married, they will be known as His Royal Highness, the Duke of... and Her Royal Highness, the Duchess of... If, for some reason, William were to refuse a title, then Catherine would become HRH Princess William of Wales. Early speculation is that Prince William's title could be Duke of Clarence, Duke of Cambridge, Duke of Connaught, or Duke of Sussex.
What's on everyone's minds, of course, is whether Catherine Middleton can someday become queen. The answer is yes: upon Prince William's hypothetical ascension, which would happen only upon Charles' death, he would become King William V and Catherine would become his queen consort, known as Queen Catherine. She would be crowned and anointed at the coronation ceremony as the queen mother was (the queen mother was Queen Elizabeth, wife of George VI--they are the parents of the present queen).
When William's father Prince Charles succeeds to the throne, the expectation is that he will make William The Prince of Wales, hereditary title of the eldest son of the monarch, but there is no automatic succession. It would only be renewed if Prince Charles wished it to. As heir apparent, William would also become the Duke of Cornwall, and receive the annual revenues from the Duchy of Cornwall. Catherine would take over Camilla's current title, Duchess of Cornwall.
And how about Harry? He will simply be the king's little brother. His name and title would not change. William's potential throne would go to his own eldest son, even if he and Catherine have a daughter first. But if the future duke and duchess only have daughters, the eldest daughter becomes queen, just like the present.