It would suck if it turns out that Jarmusch did steal the Broken Flowers script. But I guess if that happens I can still be a huge fan of Stranger Than Paradise, Down By Law, Mystery Train & Night On Earth, since their originality is uncontested. Until proven otherwise, I am going to assume that Broken Flowers is a Jarmusch original. It certainly bears a lot of Jarmusch signs: quirky characters, dry humor, a protagonist who does not speak much, a blend of cultures (Ethiopian neighbor & soundtrack), a thin plot, a road trip, scenes with unpredictable beginings & endings.
Good luck to the truth.