The Poisonwood Bible has never been made into a movie, and one has not been planned at this point. There are no movies in preproduction that have been announced to the public.
Interestingly enough, not only is there no movie version of The Poisonwood Bible, there are no movie versions of any of her books! I did find a similar movie though, called Nowhere in Africa. It is about a group of Jews in World War II that flee to Kenya. They struggle similarly to the characters in the Congo. It is about a clash of cultures.
I think that movies made from books are an excellent idea. I subscribe to the belief that a movie should not be exactly like the book, but should maintain the spirit and theme of the book.
Barbara Kingsolver, during an interview with News24 (a South African media outlet), stated that she was working on film adaptations of the Poisonwood Bible and Prodigal Summer as well as writing another novel that is still unnamed. The interview took place on on February 16, 2015. In the interview she said she is personally working on the film adaptation to keep the integrity of the book intact. I have posted the link to the interview below.
There are dozens of fan trailers on YouTube; however, the movie is still in the writing phase. As far as I can tell, no studio has purchased the rights to the book or has signed a contract to use Ms. Kingsolver’s screenplay when it is completed. Since the announcement interview took place in February of this year, it will still take her considerable time to write the screenplay and market it out to studios. I wouldn't expect to hear of any movie production aspects such as the name of the director, casting information, or filming being underway until the end of 2016 (at the earliest); more likely, it will be later in 2017.