The Wallabies are due to name their 30-man squad on Thursday, but Giteau broke ranks on Wednesday to suggest his omission in a cryptic message on the social networking site.
"Thank u ballboys, thank u linesman? That's it..." he wrote.
Giteau's place in the squad has been uncertain after he had fallen out of favour since the Wallabies' shock loss to Samoa last month. The veteran of 91 Tests has not been included in the match-day squad since.
Should Giteau be left out of Australia's squad it could spell the end of his international career with the 28-year-old due to join French side Toulon after the World Cup.
Australia coach Robbie Deans was due to inform players on Wednesday whether they were in the squad for the World Cup, which begins in New Zealand on September 9.
Giteau's tweet is not the first time an Australian sportsman has used Twitter to reveal they had been left out of international duty.
Two years ago Test cricket opener Phil Hughes tweeted he had been dropped on the morning of the third Ashes Test in England.
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