Midnight Sun isn't the fifth book in the Twilight saga, it follows the exact same story line from first to fourth book in the perspective of Edward. So its not a new story, just a different perspective.
Also, someone leaked it online and Stephenie Meyer felt violated and betrayed so she put the book on hold, saying she was so hurt that she would no longer continue with the book and would not be able to write it properly that all her anger would show through so she said she'd wait a bit and MAYBE go back to it.
I heard she may release it in 2010 after everyones forgotten about the leaking of the original manuscript, but who knows. It's not definite. It may never come out.