The Amazing Website of Kavalier & Clay (a very dedicated Chabon fansite) decided to just do the obvious and ask the man himself. Surprise! It's true: "I've been hired to do some revisions to an already strong script by Andrew Stanton and Mark Andrews. I wrote my original screenplay The Martian Agent back in 1995 because I wished I could do [Edgar Rice] Burroughs's Barsoom. So this is pretty much a dream come true for me."
Back in January, Stanton was pretty candid about how difficult the story was to adapt, but insisted they were past the writing stage, and into pre-production and casting. Clearly they've backtracked a little, but can anyone really be worried that they've stopped work to bring on Michael Chabon? This is a win-win situation all around, particularly since Chabon still hasn't managed to see any more of his work on the big screen. If John Carter spawns a Kavalier and Clay film, how happy the world will be!