There isnt a lot of information out there at the moment, but heres a quick rundown of the various whispers and rumors about the project.
Its essentially a redress of Disneylands Submarine Voyage attraction along the lines of turning Alien Encounter in Stitch. Most of the big set pieces will remain: the graveyard of ships, Atlantis, etc. Most of the moving animation the lobsters, the fish, etc. will be removed to save on maintenance costs. In there place are going large boxes which allow for the projection of computer animated images of Nemo & Friends. Theres no word on the story line yet, other than lets go explore the ocean with Nemo. There are rumors a lot of people are working on this project (the only big attraction WDI has going right now) and so there have been a lot of egos stepping over each other.
Its expected this attraction will open for the summer of 2007 as Disneylands very successful celebration of WDWs marketing event draws to a close. In the past Disneyland had opened one major attraction or show every year to keep all us locals interested in going to the park. For a while there was some interest in returning to that way of running the business, but it seems other people now have other plans for the money.
The second big rumor is that these effect boxes will also be used for the new Nemo attraction coming to The Living Seas, Phase Two. This ride will use the store bought, staple-to-the-floor, Omnimover-style ride system used by Disneylands Buzz Lightyear laid through the old pre-show areas of the pavilion. How close the two attractions will be to each other is unknown, however, Disney builds everything these days in multiple copies to spread out the costs over several parks. Id suspect the two shows will be roughly the same. If this attraction uses the same show elements as the Disneyland counterparts, it too could open in the summer of 2007.
Remember these are only rumors. Disney is a huge corporation filled with all sorts of competing interests. Just because something is in the works today doesnt mean plans wont change tomorrow.