As much as we love Stephenie to finish Midnight Sun, I don't think it will ever be completed. I believe Stephenie has stated that on her website. I believe Midnight Sun is a more powerful story of Twilight told in Edward's point of view...but unfortunately we will never read it. Reading the many chapters that Stephenie displayed on her website, you can tell the struggle Edward went through after he met Bella. He didn't know that Bella would have such an impact on his life.
She's going to finish it, but no one knows when. Stephanie hasn't put a due date to Midnight Sun. I've read everything she's written so far in Midnight Sun,if I find out when she's going to be done, I'll post it.