Giteau confirmed his 92-test career was all but over when he posted a none-too-cryptic "Thank u ballboys, thank u linesman! That's it!" message via Twitter after discussing his future with head coach Robbie Deans yesterday.
Toulon-bound Giteau's omission from the 30-man squad had already been forecast despite reservations from a host of Wallabies legends who argued his experience and versatility was essential.
However, Rocky Elsom's sacking as captain had never been envisaged although his form and leadership has hardly been compelling since the Wallabies opened their World Cup build-up with a shock loss to Samoa.
Horwill, a lock who led the Reds to their inaugural Super title last month, was told of the leadership change in South Africa on Monday. It is understood the Wallabies selectors opted to demote Elsom after the All Blacks retained the Bledisloe Cup with a 30-14 victory at Auckland on August 6.
Elsom was typically stoic as the squad were introduced inside a hangar at Sydney airport and magnanimously wished his successor well while pledging his unconditional support.
Although Elsom's downgrade was unexpected - the team were told just an hour before today's formalities got underway - he has struggled to impose himself on the blindside flank after managing just one appearance for the Brumbies leading into the Samoan debacle and condensed Tri-Nations schedule.
Horwill's appointment ensures Deans will not face the embarrassing scenario of potentially having to drop his skipper once the World Cup begins on September 9.
It also raises the possibility of form blindside Scott Higginbotham earning more game time in his specialist position while Wycliff Palu and Radike Samo contest the No 8 berth.
Deans was at pains to emphasise Elsom, the captain he appointed when his tenure with the Wallabies began in 2008, remained an integral member of the team's leadership group alongside Horwill, halfback Will Genia and flanker David Pocock.
"We believe it's in the team's and Rocky's best interests. He's working hard to get back to full fitness, he's making progress, he
can now concentrate on getting himself in the right shape without having any peripheral demands."
Deans said he was relieved at how Elsom reacted to the end of his 24-test reign as captain.
"It wouldn't have been his preference but it was made easy by his response. It was impressive and spontaneous. He simply said: 'Whatever's in the best interests of the team'."
The other talking point of the team's composition was the decision not to include a specialist back-up to Pocock, one of the world's leading openside flankers.
His durability becomes even more important now considering Western Force No 8 Ben McCalman will deputise if Pocock is unavailable or being rested.
Deans resisted the urge to include openside Matt Hodgson, explaining: "It's a reflection on how the game's evolving and being adjudicated."
All Blacks counterpart Graham Henry is likely to employ a similar philosophy and not select specialist cover for Richie McCaw when his 30-man squad are named in Brisbane next Tuesday.
Although Giteau absorbed the bad news from Deans, six wounded Wallabies previously unavailable for the Tri-Nations were included after passing fitness tests yesterday.
Palu (shoulder/knee) is selected on trust after barely playing over the last two seasons, five-eighth Berrick Barnes (concussion) contributed to Giteau's omission while centre Rob Horne (elbow), hooker Tatafu Polota-Nau (knee), prop James Slipper (ankle) and wing Drew Mitchell (ankle) also made the cut.
Palu is unlikely to play before the Wallabies' Cup campaign opens against Italy on September 11, but the others are candidates for the Australian Barbarians side who play Canada on the Gold Coast on August 26, the night before the Tri-Nations finale between the Wallabies and All Blacks in Brisbane.
