And that's not all. William also becomes Earl of Strathearn and Baron of Carrickfergus, making Kate a countess and baroness as well.
There had been local speculation in the days leading up to the wedding that the city of Cambridge would be involved in the new titles, typically reserved for younger members of the royal family.
The Guardian notes that Prince William is the 10th royal family member to be named an earl, marquess or duke of Cambridge, but the monarchy's relationship with the city has been rocky in the past:
Relations between Cambridge and the monarchy have been about as stable as the average royal marriage (present company excluded, of course). In the Reformation and in the British revolutions of the 17th century, Oxford was always seen as the loyal university, Cambridge as the stroppy one. There were puritans in both universities, of course, but Cambridge puritans always seemed the noisier and more offensive and in Emmanuel College, Cambridge developed a veritable factory for the production of preachers intent on poking Stuart kings in the eye.
The last time a Duke of Cambridge was instated was 1801 (Prince Adolphus and his son Prince George held the title). It was first created in the 17th century.