"Brad has been selected on the back of some fine domestic performances this year for New South Wales," Australian chairman of selectors Andrew Hilditch said.
"We see him capable of batting everywhere in the order and he gives us massive flexibility for the Cup.
"In addition, he provides us with cover as a second wicketkeeper should Adam Gilchrist become injured during the tournament."
Haddin is currently the leading run scorer in the domestic one-day competition, with 406 runs from 9 matches at an average of 67.66 and a strikerate of 104.10.
Tait's selection ahead of Clark, who was the star bowler for Australia during their 5-0 Ashes series whitewash, was a tough decision according to Hilditch.
"Shaun has been selected and it is fair to say it was a difficult decision to make on Stuart, who has been part of the squad this summer," Hilditch said.
"Shaun's pace will give us extra firepower during the power plays and at the end of the innings and adds variation to the pace attack for the World Cup."
Clark and White unlucky
Clark was dropped from the one-day tri-series team, but came back strongly in the domestic Pura Cup competition by taking a hat-trick in a 10-wicket match haul for NSW in their game against Western Australia in Perth last week.
|Leg-spinner Cameron White didn't make the cut|
The other notable omission from the 15-man squad was Victorian leg-spinner Cameron White, who played in the tri-series against England and New Zealand without a great deal of success.
"Cameron is extremely unlucky to miss selection in the squad. He is a young man with a bright future for Australia," Hilditch said.
"We think he'll only get better with age. The reality is that since Cameron was picked in the squad, Shane Watson has returned and Brad Hodge has seized his opportunity, meaning Cameron has missed out."
Selectors also named injured all-rounder Andrew Symonds, who is recovering from surgery on an injured bicep muscle, after consulting team medical staff.
"After lengthy discussions with the medical staff Andrew has been selected," Hilditch said.
"His recovery from surgery has gone as well as could have been expected and given his importance to the team and the balance of the team it was felt that the risk of picking him so soon after surgery is warranted."
Australia are the defending World Cup champions and are going for three in a row after beating Pakistan by eight wickets in the final in England in 1999, and defeating India by 125 runs in the final in South Africa in 2003.
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"We have been planning for the defence of the ICC Cricket World Cup for two years," said Hilditch.
"We've been pleased with the team's preparation and extremely pleased with the victory in the recent ICC Champions Trophy.
"This squad represents a balance of experience and youth and is capable of winning the ICC Cricket World Cup for the third time in a row."
The Australians open their World Cup campaign against Scotland in St. Kitts on March 13.
15-man Australian squad: Ricky Ponting (captain), Adam Gilchrist (vice-captain), Nathan Bracken, Michael Clarke, Brad Haddin, Matthew Hayden, Brad Hodge, Brad Hogg, Michael Hussey, Mitchell Johnson, Brett Lee, Glenn McGrath, Andrew Symonds, Shaun Tait, Shane Watson.