This looks absolutely delightful! I can't wait to see it.
I have to say, I read The Phantom Tollbooth as an adult, rather than as a kid, and kind of feel like I missed my window to really fall in love with this book. Which is odd, because I adored Roal Dahl's books, and as an adult, I'm a huge Terry Pratchett fan. This story is right up that alley, yet somehow I just did't connect with it.
I felt somehow that it would make a better cartoon than a book, which is something I usually never think. But it just so happens that my hero Chuck Jones thought so too, because he made a cartoon version in 1970. It looks just like his Rikki Tikki Tavi feature. See it on YouTube here: http://www.youtube.com/watch?v=Llg5VODW6n4