While Marvel Studios has yet to officially announce a Doctor Strange movie, it's probably one of the worst kept secrets in Hollywood that the film is rapidly moving forward. For instance, it's already been revealed that Scott Derrickson has been signed on to direct the film.
With that fact in mind, Derrickson just made a rather interesting tweet, which suggests he's officially begun work on Doctor Strange. Derrickson just tweeted a spooky image of Mickey Mouse, along with the text, "I now work on the Disney lot."
I now work on the Disney lot. http://t.co/WqE39H6wvA— Scott Derrickson (@scottderrickson) October 5, 2014
Now, it's possible that Derrickson is working on the Disney lot for some other reason, but we're not aware of any other Disney projects he's working on. It's more than likely his tweet means that Doctor Strange has reached the point where the cast is being finalized and pre-production work is underway.