There's a new movie of The Phantom Tollbooth coming out in a couple of years, apparently. The other attempt, which I think was in the 70s, was rather hit-and-miss (though it did have the awesomeness of Mel Blanc...)
It's planned for 2013 according to the DataBase of Movies on the Internet. I hope it turns out to be good.
I saw the 1970 animated version a few years back, and it wasn't the greatest adaptation in the world, but yeah, Mel Blanc.