The portrait was shot by Mario Testino, again, in the style of the last shoot he did with Princess Diana. The clean, white shirt is the motif he chose for her too, and is able to communicate formality and informality, warmth and distance, both men and women all at the same time. Very smart, I think.
In addition, the website of the wedding has this to say about the programme,
"The Official Programme includes the full Order of Service (as used by guests in Westminster Abbey), a personal message of thanks from Prince William and Miss Middleton, and a photo of the Couple taken by photographer Mario Testino at the time of the Couple’s engagement. The Official Programme features Prince William’s recently updated Coat of Arms on the front cover and Catherine Middleton’s new Coat of Arms on the back cover. The Order of Service includes full details about the processions, music, hymns, prayers and readings. The Wedding Service follows the Book of Common Prayer (Alternative Services: Series One). There will be one Lesson during the service, which the Couple have asked James Middleton to read. "I am a bit confused as to why the 'Couple' is referred to as a proper noun, but I intend to consult with the chief sub on the magazine immediately in this important issue! Enjoy the portrait.