Well, I'm failing to think of any comic writer who has successfully made the jump into writing movies, Frank Miller aside, and outside of David Benioff and William Monahan, I can't even really think of many novelists who have found work as screenwriters.
If you're only talking movies then J. Michael Stracznyski (Demon Night, Othersyde and Tribulations) and Neil Gaiman. If you're including TV then Paul Cornell (countless Doctor Who books which were usually fantastic), George R. R. Martin, Neil Gaiman again and countless others.
I was speaking more in reference to making the jump to movies, not television, but thanks. (How in the hell
did I forget Straczynski? I'm really looking forward to Changeling