Over on the IMDB message board for “John Carter of Mars” is a thread in which the OP says John Favreau has said the JCOM project is on indefinite hold because Favreau had to take on the Iron Man movie project on the SciFi Wire (at the SciFi Channel site).
I was able to confirm this, as the May 8 edition of Science Fiction Weekly has this quote from Favreau:
What I can’t figure is why the loss of Favreau means Paramount is putting the project on hold. It’s not like he’s Speilberg or anything. The quotes he was giving when he took the project on had all kinda red flags for me, and I’m kinda glad he’s moving on to Iron Man. Why can’t they slide in someone else? (Kerry Conran woulda been perfect.)