Unfortunately, I doubt there will be a "west-coast Castaway Cay." The Bahamas are abound with undeveloped islands which they will happily lease to companies or individuals... but the West Coast has very few islands, especially undeveloped ones. It is more likely that Disney will convert part of the largely undeveloped coastline of the Baja peninsula into their "private oasis." But that's just a guess, though.
And as the poster above said... the first ship arrives in 2011!