The Independent reports that Mendes beat Martin Scorcese to the punch on this project. Despite the one big flaw in Marty's The Age of Innocence (the narration) it's still an excellent film. I'd trust Scorcese over Mendes, I think, but after Revolutionary Road I can see the Lydgate/Rosamund plot being really devastating under the latter's direction. Anyway, I recommend the Independent article, as it's a lovely example of how to write literature and art through the medium of journalism without robbing the subject of emotion and subjectivity.
Sam Mendes has signed a two-year deal with Focus Features that includes two projects, one of which is a new adaptation of George Elliot’s Middlemarch. Better yet, Andrew Davies has already submitted a script for the film. So we’ll see this big-screen Middlemarch in either 2010 or 2011.
More discussion of the matter here and here. And the casting discussion rages on at imdb.