I had no idea... and I'm glad Shanley, the director, turned her down for the role, and instead gave it to someone who could use the work (and the money) in Viola Davis.
Plus, having Oprah co-star would likely have changed the dynamics of the film's marketing and presentation. In essence, I think it would have become more about Oprah's involvement in the project, given her immense popularity, which may have been to the film's detriment.
Although, an argument could be made that, casting her in the role of "Mrs Miller" could have added to the film's financial bottom-line - her popularity might have meant a higher volume in ticket sales... or not.
On Tuesday's episode of her daytime talk show, Oprah Winfrey revealed that she had asked to audition for a role in the film "Doubt," but was turned down by the director. Winfrey told the movie's star Meryl Streep -- who appeared on the show -- that she asked to audition to play "Mrs. Miller" because she felt a connection to it and wanted to work with Streep. But she couldn't convince director John Patrick Shanley to give her a shot. The role ended up going to Viola Davis. She received a Best Supporting Actress Golden Globe for her emotional scene in which Mrs. Miller breaks down after hearing her son may have been molested by a priest.
She apparently still wants to perform; so I won't be surprised to see her in something in the near (or distant) future, especially as she might be putting the breaks on her TV show after the 2011 season.