GhibliWorld was the first to break news on the US cast for Ponyo on a Cliff by the Sea this past week, but there seems to be some controversy on the matter. Disney has still not made any official announcement on the matter, and Anime News Network claimed that the story is false, based on a rumored test screening in Pasadena.
As with all things involving the internet, we must take these with a grain of salt. But I haven't been aware of GhibliWorld peddling falsehoods. They tend to get their facts and figures straight, even if it means ruffling a few feathers (which is unfortunate). In any event, until I hear official word otherwise, I would expect this to be the American casting for Ponyo.
This is where the Ghibli Freaks all try to discern Disney's plans for the movie, whether they will seriously promote Ponyo, or bury it to a quick death in the art-house theatres. While I'm hopeful it will be treated better than poor Howl (202 screens on opening week, then kicked down to 37), I am just as clueless as anybody. It's really not in the business of Hollywood studios to promote their rivals, after all. Even if they're guaranteed a cut of the action.
As with all things, we will simply have to wait and see.