It's both a new art form and a new PR tool: the book trailer, a subspecies of "vid-lit," so called after VidLit, a company that makes videos for books. Here's an example of a book trailer (not by that company, as far as I can tell), for James Howard Kunstler's novel, World Made By Hand, which certainly encourages the idea of a "movie based on." And here are several more examples of book trailers. Less-slick are Steven Hart's Thrill-a-Minute Skyway Cam video, apropos of his book, The Last Three Miles: Poltics, Murder and the Construction of America's First Superhighway and Gayle Brandeis's video for her new novel, Self-Storage. As I have a novel coming out next year, I'm interested in the form, but I'm also enchanted by the possibilities of incorporating sound and image (both still and moving) to fiction itself--- especially flash fiction. It's a very different concept than dramatizing the work, as in a movie-like trailer. Neither is it anything like an almost purely visual feast (e.g., Tufte's video about his sculpture, "ZZ Smile"), or even, as Internet film-maker Nick Askew does, a series of mini-documentaries on www.soulbiographies.com.
P.S. Here's some ancient history: the 2005 National Public Radio piece on vid-lit.