I don't know, they had him working on a Wonder Woman script forever, and apparently couldn't come to terms on how to approach the character for a movie
On a parallel note, from an interview
http://www.comicbookresources.com/?page=article&id=38239 here's Whedon's take on what makes Marvel's characters different from DC's and why they worked for launching an ensemble movie-
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"It's enormously difficult to take very disparate characters and make them work, and DC has a harder time of it than Marvel because their characters are from a bygone era where characters were bigger than we were," Whedon said. "They've amended that, but Marvel really cracked the code in terms of, 'Oh, they're just like us!' So a dose of that veracity that Marvel started with 'Iron Man', I think you need to use that as your base."
Is he right? Wrong?
Strangely enough, DC has one of the most not super but "human" characters in Batman. Is he from a bygone era? Or is the contemporary take on the character completely relevant for toady? I think the later.
What Whedon describes as Marvel having a "base" to build on in Iron-Man, DC had in the Dark Knight, they just refused to recognized it as their base, and saw it and the rest of their franchise as separate.
I think this was their biggest mistake.