According to the Viz catalog, the Film Picture Book for Ponyo on a Cliff by the Sea will be released here in the US sometime during Spring 2009. It runs 152 pages and should cost $19.99.
This should give us a good clue as to when Disney/Pixar will release the US version of Ponyo in theatres. I had expected a summer release, but spring may be better. With much less competition against rival studios (and Disney's own product), there will be a stronger incentive to properly promote the film. I'd certainly hope they would (finally) give a Studio Ghibli movie the proper exposure it deserves. 202 screens (Howl) is just insulting.
Maybe I should send a couple emails to Disney and Ponyo's US producers, and see if they'd answer a few questions for us. We'll see. The election season is nearly over, so I'll actually have some spare time to devote to these things again.