The confirmed worlds are:
Fabled Country Side - Beauty and the Beast
Beast was one of your teammates at the end of the first installment, and Belle was one of the seven princesses of heart, but this time, we get to go to their world, which is fully playable this time around.
Ancient Greece - Hercules
Greece was not really an explorable world last time. It was just one huge colloseum used for battling purposes and nothing more. It's fully playable this time around.
Agrabah - Aladdin
Agrabah is returning in full form, apparantly due to its extreme popularity with Final Fantasy fanboys. Agrabah is expected to be even more expansive this time around.
Ancient China - Mulan
No one knows how Mulan will figure into the plot, but video clips with Mulan as a playable character confirm its existense in the game. Its thought that this segment will closely resemble the movie's plot with minor deviations for the sake of the Kingdom Hearts world.
The almost-confirmed, highly rumored worlds are: (no video/screencap confirmation available to date)
Nottinghamshire - Robin Hood
Pride Rock - The Lion King
Camelot - The Sword in the Stone
Prydain - The Black Cauldron
Rumored/Long-shot possibilities:
Victorian London - The Great Mouse Detective
Indian Jungle - The Jungle Book
Non-Disney Worlds:
An original world called Deep Dive is said to be an integral part of the game. The name comes from a secret video available on the first Kingdom Hearts Japanese version.
At least one Final Fantasy World is expected to make an appearance. Worlds/characters from the DragonQuest series are also being considered.
Characters without worlds:
Pete, Goofy's archenemy, is said to be one of the major storyline villains in Kingdom Hearts 2.
Jack Skellington, due to overwhelming popularity, will make an appearance, even though his world looks to be dropped from the second game.
Worlds that are rumored, but seem to be abandoned:
Atlantica - The Little Mermaid
Ariel seemed to be in distress in Kingdom Hearts: Chain of Memories, but since in the series she is not one of the princesses of heart, the creators obviously don't feel the need to bring her world back for a visit. Its rumored that fanboys were not overly enthusiasitc the first time around.
Colonial America - Pocahontas
This was actually the first world to be rumored almost a year ago, along with Mulan. Too many female leads for this boy-targeted game? Whatever happened, Pocahontas seemed to be scrapped from the project, probably to leave some major plot points for a third installment.
R[
The game might not come out in September though, so do not get your hopes up for it to be released then. As of now, there is no release date. Not untill Square (if they ever decide to do anything) mentions one.