If there will be something at COMIC-CON it'll probably be some art. Or if you're lucky probably some sort of proof-of-concept test.
I just checked the wiki page on Carter and filming is supposed to end in July 2010. I don't know think will show blue screen footage or incomplete stuff.
My guess is that Disney/PIXAR will follow Tron Legacy's promo-campaign . Considering that just like Tron Legacy , this is also one of Disney's big 3-d release ,they will probably want to show footage in all it's 3-d glory
As of now the release date it currently set for 2012. But don't be surprised if Disney also decides to release John Carter Of Mars somewhere in dec 2012 , should Tron Legacy become profitable.
It'll be interesting to see if WB decides to release Batman 3 in dec 2012.
John Carter Of Mars vs TDK sequel

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