i'm at work right now, trying to find robert rodriguez's phone number. Confused why he wasn't in the DGA, wikipedia helped me out and dropped this in:

Rodriguez co-directed Sin City (2005), an adaptation of the Frank Miller Sin City comic books; Quentin Tarantino also guest-directed a scene. During production in 2004, Rodriguez insisted that Miller directed the film with him because he considered the visual style of Miller's comic art to be just as important as his own in the film. However, the Directors Guild of America would not allow it, citing that only "legitimate teams" could share the director's credit (e.g. the Wachowski Brothers). Rodriguez chose to resign from the DGA, stating, "It was easier for me to quietly resign before shooting because otherwise I'd be forced to make compromises I was unwilling to make or set a precedent that might hurt the guild later on." By resigning from the DGA, Rodriguez was also forced to relinquish his director's seat on the film John Carter of Mars (2006) (at the time "A Princess of Mars" after the book on which it was based) for Paramount Pictures. Rodriguez had already signed on and had been announced as director of that film, planning to begin filming soon after completing Sin City.