It's going to be available this spring on a DVD which includes the original text (though you can also find that online, as it's in the public domain). This animated film features the voices of Martin Sheen as Arthur Square, Kristen Bell as Hex, Tony Hale from Arrested Development as the King of Pointland, and Michael York as Spherius. Joe Estevez, Martin Sheen's brother, is Abbott Square--although he's been doing tons of movies (IMDB shows 160 titles, 22 of which are dated 2006 or 2007), I haven't seen any since MST3K covered 1990's "Soultaker."
If you check out the movie website, you can register for a discount on the DVD when it becomes available.
UPDATE (March 20, 2007): Thanks to Gourami118, who points out in the comments that there are two Flatland movies. The five-star Film Threat review is of this movie by Ladd Ehlinger, Jr., which completed in January and is already available on DVD in a limited run of 1,000 copies. Call this one "Flatland: The Film" (based on its website), while the Martin Sheen one is "Flatland: The Movie."