Despite Disney and Ghibli being close, this isn't an official thing. But it's not strictly a "fan" pipe dream, either. John Ramirez (whose art we showed off this morning) is an artist at Disney, where he hasn't just worked on movies like The Lion King and Aladdin, but as a theme park designer as well. In his spare time he's also a big Studio Ghibli fan, which brings us to this idea for a Totoro ride.
The design of the ride, the way it hits all the movie's key moments, the fact you sit in a giant acorn, the way it uses technology to grow a giant tree and make the Catbus disappear... all signs of a man who clearly knows his shit, about both Totoro and theme park rides in general.
For what it's worth, while this isn't an official thing yet, that's not for want of Ramirez trying; he tells Kotaku that he's spoken to Hayao Miyazaki a number of times about Disney building this ride somewhere, and while he's been knocked back each time says he'll keep asking "until they bar me from the studio or until he says yes!"
While we're on the subject, if you missed it, here's another fan's idea for a whole Studio Ghibli theme park.