This Is where the press do not know what they are talking about, A commoner is someone who Is not a soverign , Duke , Earl or baron , thus till recently had an automatis right to sit in The House of Lords .
So In fact The Lady Elizabeth Boyes- Lyon was the first commoner to marry In the Royal family In recent times, later to become the Queen Mother . Diana Princess of Wales was also a commoner when she married .
And In fact Prince William Is offically a Commoner until he Is given the status of a Dukedom etc .
http://en.wikipedia.org/wiki/Commoners_in_the_United_Kingdom