Brandon in June, 2009 on possibility of Mistborn movie:

"It is a small production studio, so nobody you'd recognize. The producer is a fan of the Mistborn books who has some credentials in independent films, and who has impressed me with his treatment of the books and his determination to make the film. This individual is starting a production company to focus on the film. We're in the contract stages now, and once that is done, I can be more specific.

It's not like the Alcatraz movie, which was optioned directly by a studio. Because of that, the Mistborn movie is probably a lot less likely to happen--but, the hands it is in are quite good. Anything having to do with Hollywood is a long-shot in the first place, so (after meeting with the producer) I decided that I'd rather take the slightly more unlikely chance in exchange for the opportunity to work with someone I felt understood the books."

Mistborn Movie Guide: Both Sanderson and I agree that the chances of this movie being made and ever see the screen are very slim. However, now that the books have been optioned, we may begin to see the project pick up steam.
Discussion of this comment here at the Time Waster's Guide.