From an interview with Terry Brooks:
Still very vague, but seeing as Shannara is one of the longest-lasting fantasy series, I reckon it might just happen.
Q: I can't understand why Hollywood has never adopted the Shannara books, or indeed the Knight of the Word books. Have your books been optioned? Is there any chance of a movie?
TB: I have sold options to various movie studios on both Shannara and Magic Kingdom over the years, as early as the late seventies. But the experiences were not pleasant, and after awhile, I withdrew the invitation to even talk about it. That changed with the production of Harry Potter and Lord of the Rings, both of which demonstrated in no uncertain terms how far special effects had advanced and how good movies could be made based on solid story telling. So earlier this year I optioned the Shannara series to Warner Brothers. There is reason to think that they intend to do something important with it. But, as usual, it's wait and see time.
Still very vague, but seeing as Shannara is one of the longest-lasting fantasy series, I reckon it might just happen.