In an article at The Daily Beast about Avatar and 3D, the site mentions the following:
The Avatar factor could make the difference on Cowboys and Aliens, a DreamWorks project to be directed by Jon Favreau that starts shooting in July.
The movie will be based on the Platinum Studios Comics graphic novel written by Fred Van Lente and Andrew Foley. The sci-fi Western explores what would happen if the traditional Old West enemies — cowboys and Native Americans — found the prairie attacked by aliens in mid-1800s Arizona.
Alex Kurtzman, Roberto Orci and Damon Lindelof wrote the script.
Source: The Daily Beast