The problem is (as it often is): Politics.
Wizards of the Coast (then TSR) sold the entertainment (read: Movie and TV) rights to Courtney Solomon, who was lucky enough to have a billionaire bankrolling him at the time. Courtney is currently amusing himself with SyFy/direct-to-dvd horror movies at After Dark Films, so there's no real desire for him to work with WotC.
Some properties related to DnD (like Dragonlance) are getting pushed by Cindi Rice and John Rosenblum over at Epic Level Entertainment, but they're only working with titles that don't need licensing from DnD. Senior management at TSR/WotC have a history of letting licensing opportunities slip through their fingers - they famously passed on working with the X Files because nobody had watched the show. The main champion WotC had for entertainment deals, Scott Rouse, stepped down at the end of last year.
The main issue is that Brian Goldner, the CEO of Hasbro, decided that all Hasbro and Hasbro subsidiary properties are to be internally developed - and has set up a new studio division to do so. GI Joe was their first big feature, and they have Battleship (directed by Peter Berg from Friday Night Lights), Monopoly (directed by Ridley Scott), and Stretch Armstrong (starring Taylor Lautner) on the way, with Magic the Gathering in development. They've set up distribution deals with Universal and Discovery Communications, and like Lucasarts they're recalling all other properties where they can so that they don't have to work with outside producers (Don Murphy, who produced the first two Transformers movies, commented that Goldner is "bogarting everything").
So until Hasbro decides to work with external producers again, WotC gets a new entertainment champion, and Courtney Solomon decides to give up the rights, there's probably not going to be any DnD movies in the near future.