According to io9, James Cameron may have spontaneuously come up with the concept for Avatar after reading Poul Anderson's 1957 sci fi novella Call Me Joe, and then conveniently forgetting that he read it:

Lik Avatar, Call Me Joe centers on a paraplegic — Ed Anglesey — who telepathically connects with an artificially created life form in order to explore a harsh planet (in this case, Jupiter). Anglesey, like Avatar's Jake Sully, revels in the freedom and strength of his artificial created body, battle predators on the surface of Jupiter, and gradually goes native as he spends more time connected to his artificial body.

Cameron is no stranger to accusations of plagiary.  After Terminator's release, writer Harlan Ellison sued the production company for ripping off two episodes he wrote for The Outer Limits.  The company settled and Ellison is acknowledged in the film's credits.

As far as Avatar is concerned, at least Cameron had the creative insight to change the name of the title from Call Me Joe.  Sounds like a biopic of Joe The Plumber, who would undoubtedly be played by a bald Tim Allen