I haven't seen the special feature yet, but the Little Mermaid ride was originally planned for Euro Disney. That park's financial problems lead to the "Phase 2" plans being dropped when the park opened. The ride was announced as part of the "Disney Decade" plans for Disneyland. It would have been a new attraction and located in the old Fantasyland Boat Lagoon. But here too, the "Disney Decade" plans were scrapped almost at the same time they were announced.
actually I don't want to get anyones hopes up but I was reading the 2007 unofficial guide and theres speculation that disney might be opening a little mermaid ride at the magic kingdom expanding off of the area dedicated to her in fantasyland...here's hoping!