Doom movie sequel?

It's not so much in the works yet but Karl Urban and Rosamund Pike have alread signed on for Doom 2. They probably weren't doing too much anyway. Dwayne "The Rock" Johnson hasn't signed on for anything yet, probably because he doesn't want to commit to a sequel if the first movie bombs itself out of existance (which is to be expected with a video game movie, who rememebrs the mario bros movie? exactly).