There was an interview posted on her site in 11/16/09 where she stated that she would eventually finish and publish Midnight Sun, but that she didn't know when and that it wasn't her current project.
Here's the link to the complete interview:
http://stepheniemeyer.com/nm_movie_qanda.html
She talks about it aaalllll the way down at the bottom, last question.
(I know way more about this stuff than is healthy, I'm sure.)