The two have been a couple since William's university days, and speculation about their future is a continuing subject of gossip and speculation.
With grandmother Queen Elizabeth II addressing the new second lieutenants and father Prince Charles taking the salute, William was one face in a crowd of cadets participating in "passing out" ceremonies at the Royal Military Academy, Sandhurst, following a 44-week commissioning course.
William's younger brother, Prince Harry, was commissioned at a similar ceremony earlier this year. Both of the young royals were assigned to the Blues and Royals regiment of the Household Cavalry.
CBS News correspondent Mark Phillips says there are thought to be very different philosophies behind the royal family's decisions to send the two brothers to the academy.
For Harry, it was deemed a good way to get a rather rambunctious — and inclined to get himself into trouble — member of the family to straighten up and fly right, whereas with William, it may have just been a lack of any better ideas.
Royal watcher Victoria Mather explains to CBS News, "you always have this problem with the heir. What do you do to keep them occupied until they become king? Well, that's going to be quite a long time, and where do you keep them safe?"
To see photos from the ceremony, click here.
Keeping him safely stored in the Army seemed like the best idea, but Phillips reports, young William isn't ever likely see bullets fly past his head, or be forced to dodge insurgents in Iraq.
"He can't be sent to Iraq. He can't be sent to Afghanistan," Mather says, "they're two of the most pointless theaters of war in modern history and if he was injured, if he was killed — can you imagine the victory for the other side in terms of PR?"
As Phillips pointed out to people who keep tabs on the royal family, the presence of William's girlfriend at the event was not at all insignificant.
Wearing a bright scarlet coat and dark hat, Middleton attracted lingering gazes from the television cameras covering the ceremony.
It was the first time Kate — who Phillips says the Royals have taken to calling Katherine, which is seen as a sign of respect in Britain's highest social circle — was invited to an event that the queen herself was simultaneously gracing with the royal presence.
Rumors have been swirling in Britain about when or if Prince William might propose to his girlfriend, and her presence Friday, along with her invitation to join the queen's family for Christmas dinner, will certainly fan the flames of intrigue.
"For those who are to be commissioned today and to those who will shortly follow, a great deal will be expected of you," the queen told cadets.
"You must be courageous yet selfless, leaders yet carers, confident yet considerate... These are very special attributes, but those whom you will command and your country too will expect nothing less. My prayer for your success and safety will follow you wherever you happen to serve."