This thriller comes from the unlikeliest of sources: Steve Martin's brain. The comedian dreamt up the espionage tale, took an exec-producer credit, then stepped aside and let the experts take over. And it turns out that's not the only unusual thing about Traitor. ''I've never seen a movie where a Muslim is the hero,'' says Don Cheadle, playing a CIA operative who manages to infiltrate a band of terrorists — with unexpected consequences. ''He gets caught up and has his allegiances, his Muslim faith, tested. It hopefully will serve as a good jumping-off point to examine this issue.'' (August 29)