According to narniafans.com:
"Coming in 2006, Disney-MGM Studios Guests Can Step Behind the Scenes of 'The Chronicles of Narnia' Motion Picture.
For fans of C.S. Lewis' timeless adventure The Chronicles of Narnia: The Lion, The Witch and the Wardrobe, Disney-MGM Studios is transforming a soundstage into the frozen world of Narnia and taking guests behind-the-scenes of the movie-making magic in the film based on the book. Guests will enter the frozen world of Narnia through a giant wardrobe.
In addition to props and set pieces, guests will see elaborate creatures, costumes, and armor used in making the film.
The walk-through experience is located in Soundstage 4 on Mickey Avenue."
I wasn't able to find anything for you on the official Disney web site, I would think the opening of the walk-through would be closer to the time of the movie opening up (Christmas) so they can do some cross-promotion? Maybe they need more time to get the exhibit ready? Anyway, have fun on your trip!
