According to Deadline, Nolan is taking the script that his brother wrote, and combining it with his own original idea, and writing a new script (the movie will retain the title). Thorne is still on board as a technical consultant. As for the exact story, yesterday it was said that it will follow a group of explorers who go through a wormhole, in a story about time travel and alternate dimensions. While today, things are a bit more vague, as stating it will "depict a heroic interstellar voyage to the farthest borders of our scientific understanding."
We can only imagine that just like the early days of "The Dark Knight Rises," there will be plenty of speculation and guesswork, but either way, I think we can all agree that Nolan taking his ambition on a bigger scale is something to really look forward to. He'll spend the early part of the year as executive producer on his friend and cinematographer Wally Pfister's directorial debut "Transcendence," and of course there's "Man Of Steel" coming this summer. It sounds as if he's still putting things together for "Interstellar," and if he's doing more writing, then it might not be camera-ready just yet. But hopefully once his commitments to those two movies wrap up, this won't be too far behind.