if he is given another title like Earl or Duke like Prince Edward then she will no longer just be a commoner but a Countess or Duchess,otherwise she will have to be known as Princess William (same as Princess Michael is now)
even nobility are technically commoners but with a title. This has happened before - wit Diana and also the Queen Mum.....both minor noble commoners.
The other prerequisite is that William would have had to seek the blessing of the Queen, as well as Kate's father, before proposing. Re the Queen they call it a quota or spot technically I think....
I am wrong its just the blessing. A roiyal prince in line to the throne can marry a female of another religious persuasion, as long ashe converts by the way......
there is an over-rider to the Monarch's consent or, rather, lack of it - "Any member of the Royal Family over the age of 25 who has been refused the sovereign's consent may marry one year after giving notice to the Privy Council of their intention to so marry, unless both houses of Parliament expressly declare their disapproval. There is, however, no instance in which the sovereign's formal consent in Council has been refused"
He will probably be given a royal dukedom in order for his wife to be styled duchess, as there is no special title for the eldest son of the Prince or Wales. The eldest son of Edward VII, while he was still Prince of Wales, was given the title of Duke of Clarence (died prematurely).
shame they cant give him St Andrews given that fine institution (also my own alma mater) is where they romanced; that title went to Prince George, 4th son of George V and has come down through to the Duke and Duchess of Kent's son. Betting would favour Cambridge - or perhaps Lancaster????
Amusingly I rather suspect that an heir to the throne who's marriage was not approved of by the monarch could appeal under human rights legislation with a good chance of success.
Cambridge, Clarence, Albany or Sussex would be my guess, but definitely not Lancaster. That title is reserved for the Sovereign, a relic of the Wars of the Roses. The present holder is the Queen.