LONDON, ENGLAND - JUNE 21:  Queen Elizabeth II  walks up the staircase in 10 Downing Stree with Prime Minister David Cameron before having lunch together to mark the Duke of Edinburgh's 90th birthdayon June 21, 2011 in London, England. The Camerons have invited the Duke of Edinburgh and The Queen to Downing Street to celebrate The Duke's 90th birthday.  (Photo by Stefan Rosseau - WPA Pool/Getty Images)