If you want my opinion, I'm thinking one of them (Affleck seems to be the bigger idiot) probably mentioned something in an interview noting that their "dream" project would be to remake Butch and Sundance. That was then twisted and turned around to the point that Damon's name got thrown in and - bam - here we are, on Cinematical, praying this thing does not get made.
Damon and Affleck last shared the screen in Dogma (though I believe Matt had a small role in Jersey Girl) and are best known for their "collaborative" effort on Good Will Hunting, for which the two won an Oscar for best screenplay. Honestly, Damon has done a good job of separating himself from Affleck over the last few years, and with a few big roles coming up, it's probably in his best interest to stay away from his buddy who is, ultimately ,slipping from the spotlight. What do you think about this?