Twilight Midnight Sun is supposed to be Twilight (First book or the whole saga?) as told from Edward's point of view. Because an early draft leaked online Stephenie Meyer stopped working on it and postponed its release:
Since the Twilight Breaking Dawn movie will enter production quite soon, I think she will reconsider her position and will try to ready the novel.
Does that mean we can hope for a Twilight Midnight Sun movie? Well, I'm sure Summit Entertainment would be damn glad to extend the life of the Twilight Saga movie franchise. And I'm sure that the Twilight fandom would love it. But it's just speculation as of now...
And not sure the cast will be available: Robert Pattinson, Kristen Stewart and Taylor Lautner are loyal to the Twilight franchise, but they already have signed up for new projects (and you can't dismiss such contractual obligations on a whim). So it's not clear how the Twilight Midnight Sun movie would fit in their busy schedules.
But I guess, since Twilight Midnight Sun is not a real brand new story (we just switch from Bella's point of view to Edward's), instead of making a live-action movie with flesh and blood actors, they could go for an animated movie: Twilight fans would like it too, after all they did rush to buy Twilight The Graphic Novel.
Anyway let's wait for Stephenie Meyer to release Twilight Midnight Sun first!