The British Royal Court released information about the uniforms of HRH the Duke of Cambridge, Prince William, and HRH Prince Harry of Wales at the wedding of The Duke of Cambridge and Catherine Middleton.
From the official YouTube channel of the Royal House:
"Prince William has chosen to wear the uniform of Colonel of the Irish Guards on his Wedding Day. Prince William, who is commissioned in all three Armed Services, and who has served actively with the Army (The Household Cavalry Regiment) and with the Royal Air Force (Search and Rescue Force), chose to wear the uniform of his senior honorary appointment in the Army.
Prince Harry wears the uniform of a Captain of the Household Cavalry (Blues and Royals)."